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A  System  of  Ocular  Skiametry 

INCLUDING     SUCH     PORTIONS    OF  OPTOMETRY*  AS  [ARE 

PERTINENT  TO  THE  USE  OF  THE  "SHADOW  TEST" 

WITH  THE  PLANE   MIRROR 


BY 
ANDREW  JAY  CROSS 


Author  of  "  Dynamic  Skiascopy,  or  the  Action  of  the  Accommodation  in  Shadow  Testing, 

and  other  papers.      Member    of  the    Optical  Society  of  the    City  of  New   York 

President    of   the    American    Association    of  Opticians,    1900  to   1901 

President  of  the  Optical  Society  of  the  State  of  New  York 

1897  to  1900.      Honorary  Member  of  the  Central 

New    York    Optical    Society  and  of  the 

Rochester  N.  Y.  Optical  Club 

etc.   etc. 


IVith  Ninety-Four  Illustrations 


NEW  YORK 

FREDERICK   BOGER   PUBLISHING  CO. 

36   Maiden   Lane 

1903 


(f-^oX-thv. 


r 


COPYRIGHT.    1903,  BY  ANDREW  JAY  CROSS. 


PRESS  OF  "THE  OPTICAL   JOURNAL," 
NEW    YORK. 


OPTOiNfCTRY 
LIBRARY 


To 

Those  Opticians 

Who  Have  Shown  Themselves  Ambitious  to  Elevate  Their  Calling 

This  Book  is  Fraternally  Dedicated. 


MG7514:2 


PREFACE. 

It  is  with  some  timidity,  both  from  a  literary  viewpoint 
and  from  tliat  of  a  retailer  of  spectacles  and  eyeglasses,  and 
manufacturer  of  optometrical  devices,  that  this  book  is  offered 
for  the  consideration  of  those  who  adapt  glasses  to  the  eyes 
of  others. 

If  it  is  true  that  "necessity  is  the  mother  of  invention," 
then  perhaps  it  will  be  idle  to  offer  further  excuses  here.  The 
author,  however,  desires  to  express  his  deep  appreciation  of 
the  work  done  by  the  many  medical  men  who  have  enhanced 
the  value  of  Sir  William  Paget  Bowman's  discovery  of  the 
phenomena  now  known  as  the  "Shadow  Test."  Especially  is 
the  author  under  obligations  to  such  writers  as  Dr.  Edward 
Jackson  and  Dr.  James  Thorington  for  aid  derived  from  their 
able  monographs  on  this  subject.  But  nearly  all  medical 
writers  having,  naturally,  written  from  the  standpoint  of 
physicians,  that  "the  use  of  a  reliable  cycloplegic  is  always 
indicated,"  it  has  remained  for  opticians  to  devise  ways  and 
means  for  accurately  applying  the  principles  of  this  test  with- 
out resorting  to  the  use  of  local  toxicants  of  any  kind. 

This  small  volume  has  therefore  been  prepared  for  the 
purpose  of  pointing  out  new  and  better  paths  in  practical 
optometry,  as  well  as  the  re-blazing  of  old  ones,  and  the 
author  begs  that  all  optical  and  medical  readers,  particularly 
those  who  are  wedded  to  old-fashioned  methods,  will  bear  in 
mind  that  the  use  of  mechanical  devices  is  the  order  of  the 
day,  and  that  the  accuracy  of  the  experiments  alluded  to  in 
the  following  pages  can  be  readily  proved  by  any  one  suf- 
ficiently interested  to  attempt  their  demonstration. 
20  East  230  Street,  New  York. 
October  i,  1(^03. 


Dbraryof  the  Alamerfa 

Counuy.  Assoeiation 

of  OpU^metrists 

CONTENTS 

CH APT KR  I PaRcs  1 1  to  2^. 

A  Descriptive  Name. 

Amount  of  Optical   Knowledge   Necessary   to   Achieve    Skiametric 

Success. 
The  Value  of  Bowman's  Discovery. 
Difficulties  of  Ocular  Skiametry. 

CHAPTER  II Pages  24  to  49- 

Adequate  and  Inado(iuate  Examination-Rooms. 

Illumination:     Its  Size,  Source  and  Control. 

The  Plane  Mirror:     Its  Construction  and  How  to  Handle  It. 

Good  and  Bad  Schematic  Eyes,  and  How  to  Correct  Them. 

A    Short    Method    for   the   Reduction   and   Transposition    of   Lens 

Values. 

CHAPTER   III Pages  50  to  69. 

Optical  Principles  Involved  in  Skiametry. 

The  Shadow,  What  to  Look  For  and  How  to  See  It,  and  How  to 

Imitate  Its  Action  with  a  Card-board  Model. 

CHAPTER   IV Pages  70  to  86. 

.•\ction  of  the  Shadow  and  What  It  Indicates. 

Appearance   of  the    Sliadow    in   Regular   and   Irregular   Errors   of 

Refraction    and    Its    Action    in    Emmetropia,    Myopia,    Hyper- 

metropia  and  Astigmatism. 

CHAPTER    V Pages  87  to  97. 

Some   Theories   Regarding   the   Dulness   of   the    Fundus    Reflex    in 

Certain  Cases. 
Also    Multiple    Methods    of    Practising    Skiametry,    Including    the 

Toxic  and  Non-Toxic  Planner  of  Employing  the   Static  Test 

by   l-5()tli   the   .Amplifying  and   Fogging  Methods. 

CHAPTER    VI Pages  98  to  iii. 

Dynamic  Skiametry  and   Its   Uses   in   Mastering  Tonic  and   Clonic 

Spasms. 
The   Importance  of  Visual    Fixation   in   Obtaining   Bright   Retinal 

Reflexes,  and  the  Aid  Derived  from  Using  Independent  Points, 

Together  with  a  Few  Words  .About  Ray  Values. 


CONTENTS— Continued 

CHAPTER    VII Pages  112  to  123. 

Ocular  Muscle  Action,  and  the  Influence  of  Habit  Upon  Accom- 
modation, Convergence  and  Innervation,  with  Special  Reference 
to  Spasms,  Latent  Errors,  and  the  Use  of  Prisms. 

CHAPTER    Vni Pages  124  to  129. 

Corroborative  IMeasurements  in  Optometry  and  the  Inter-Depend- 
ence of  Objective  and  Subjective  Methods.— The  Value  of 
Mechanical  Devices  and  the  Superiority  of  the  Mobile  over 
the  Unit  Action  of  Lenses. 

CHAPTER    IX Pages  130  to  146. 

The  Use  of  Instruments  in  Skiametric  Work,  Their  Evolution 
from  Single  Lenses  and  the  Relative  Merits  of  Some  Now 
Employed,  including  a  Description  of  the  Author's  Own 
Mechanical  Contributions  in  this  Line. 

CHAPTER    X Pages  147  to  153. 

Systematic  Ocular  Examinations  and  the  Aid  Derived  from  Mak- 
ing Complete  Prime  Records. — Resourcefulness  in  Refraction 
Work  and  the  Successful  Exainination  of  the  Eyes  of  Children, 
Mutes  and  Illiterates. 

CHAPTER    XI Pages  154  to  168. 

Illustrative  Cases,  Showing  the  Comparative  Value  of  Static  and 
Dynamic  Skiametry  in  Various  Patients  of  Different  Ages, 
Occupations  and  Apparent  Physical  Condition. 

CHAPTER    XH Pages  169  to  181. 

Resume  of  Previous  Chapters  With  a  View  to  Emphasizing  the 
Salient  Points  of  Ocular  Skiametry  as  a  System. 


ILLUSTRATIONS 

Fig.  Page 

1  Parallel  rays  of  light  being  focused 15 

2  Fociised  rays  of  light  being  paralleled 15 

3  A  "Success"  burner  oil  lamp 27 

4  An  "Argand"  burner  gas  lamp 28 

5  An  acetylene  gas  lamp 29 

6  A  "Welsbach"  gas  lamp 30 

7  A    "Welsbach"   gas   lamp   with    a    large   asbestos-lined    "Cross" 

chimney    31 

8  A  small  asbestos-lined  "Cross"  chimney  for  citlier  a  "Welsbach" 

or  an   "Argand"   lamp 32 

9  A  gasolene  "Student"  lamp x^ 

10  Showing  spiral  filament  for  electric  lamp 34 

11  The  "Cross"  asbestos-covered  electric  lamp 35 

12  The  "DeZeng"  luminous   retinoscope 36 

13  Wall  bracket  for  gas  or  electric  lamp 37 

14  Section  of  the  "Cross"  mirror  showing  concave  l)ack 39 

15  The   "Cross"   Bracket    ]\Iirror 40 

16  Showing  manner  of  holding  mirror 41 

17  A  "Queen"  pasteboard  schematic  eye 43 

18  A   home-made    "smoke"   box 51 

19  Showing  refraction  by  lens  of  proper  focus 51 

20  Showing  refraction  by  too  strong  a  convex  lens 5-2 

21  Showing  refraction  by  too  weak  a  convex  lens 52 

22  Rays  of  light  emerging  unrcfracted 54 

23  Rays  emerging  properly  refracted 54 

24  Emerging  rays  having  a  conjugate  focus 55 

25  Rays   emerging   divergent 55 

26  Rays   emerging   unrefracted 56 

27  Rays   emerging  insufficiently   refracted 56 

28  Rays  emerging  properly  refracted 57 

29  Rays  emerging  with  divergency  increased 57 

30  An   Emmetropic  eye 5^ 

31  A  Myopic  eye 59 

32  A   Hypermetropic  eye 59 

33  Illustrating  a  so-called  "shadow" 61 

34  Showing  now  the  retma  is  illuminated  l)y  reflected  light 63 

35  Showing  the  source  of  the   returning  light   to  be   the   edge  of 

the    illumination 63 

36  First  half  of  pasteboard  model  for  demonstrating  the  shadow's 

action  65 


ILLUSTRATIONS— Continued 
f'^-  Page 

37  Second     half    of    pasteboard    model     for    demonstrating    the 

shadow's    action g^ 

38  Showing  why  the  shadow  moves  with  the  mirror 71 

39  Showing  why  the  shadow  moves  against  the  mirror jj 

40  Illustrating  total  refraction  in  Emmetropia j;^ 

41  Ilhistrating  a  weak  degree  of  Hypermetropia 74 

4-'     Illustrating  a  marked  degree  of  Hypermetropia 74 

43  Illustrating  a  weak  degree  of  Myopia 75 

44  Illustrating  a  marked  degree  of  Myopia 75 

45  Illustrating   Hypermetropic  Astigmatism 76 

46  Illustrating   ^Myopic  Astigmatism -(] 

47  Illustrating  Compound   Hypermetropic  Astigmatism j'p 

48  Illustrating  Compound  Myopic  Astigmatism ^^ 

49  Illustrating  Mixed  Astigmatism yg 

50  Illustrating  Mixed  Astigmatism  at  oblique  axes 78 

51  Showing  the  crescent-like  shadow  in  Spherical  Cases 81 

52  Showing    the    straight-edge     appearance    of    the    shadow     in 

Astigmatic    Cases 81 

53  Showing  the  band  observable  in  high  degrees  of  Astigmatism.  .  82 

54  Showing  an  Astigmatic  band  at  oblique  axis 82 

55  Showing  the  meridional  appearance  of  the  shadow  in  Compound 

Astigmatism    83 

56  Showing  the  double  band  in  the  "Scissors"  movement 84 

57  Showing  a  case  of  "Irregular"  Astigmatism 84 

58  Showing  a  case  where  "Cortical  Cataract"  is  present 85 

59  Showing    the    shadow    as    it    sometimes    appears    in    "Conical 

Cornea"    85 

60  Showing  why  the  retinal  illumination  is  large  in  marked  errors.  88 

61  Showing  why  the  shadow  moves  slowly  in  marked  errors 89 

62  Showing  why  the  shadow  is  duller  in   Myopia   than   in   a   like 

degree  of  Hypermetropia go 

63  Showing  relative  size  of  retinal   illumination   in   high   and   low 

degrees  of  Myopia 91 

64  Showing  the  optical  principles  of  Penumbra 91 

65  Showing  optical  principles  of  Penumbra  doubled 92 

66  Showing  the  interference  of  Penumbra  in  shadow-testing 93 

67  Showing  emerging  rays  being  bent  to  a  focus  by  a  trial  lens.  ...   100 

68  Shov/ing  emerging  rays  being  bent  to  a  focus  by  accommodation  loi 

69  Showing  how  accommodation  can  be  made  to  absorb  a  ciliary 

spasm   103 


ILLUSTRATIONS— Continued 
Fig.  Page 

70  Showing  the   assistance  offered   by   niuUiplc   fixation   points   in 

Dynamic    Skiametry 106 

71  "The  Cross"  Fixation  Stand 107 

^2     Showing  primary  position  for  fixation  card 108 

73  Showing   normal    balance   between    Accommodation    and    Con- 

vergence in   Emmetropia 113 

74  Showing  relative  Innervation  necessary  to  balance  Accommo- 

dation and  Convergence  in  Emmetropia 114 

75  Showing  lack   of  balance   between   Accommodation   and    Con- 

vergence  in   Hypermetropia 115 

76  Showing  the  unequal  Innervation  required  to  balance  Accom- 

dation  and  Convergence  in  Hypermetropia 115 

'•J     Showing  lack   of  balance  between   Accommodation   and   Con- 
vergence in  Myopia 1 16 

78  Showing  the  unequal  Innervation  required  to  balance  Accom- 

modation and  Convergence  in  Myopia n6 

79  Showing  the  optical  principles  of  mobile  lens  action  produced 

by  changing  the  relative  position  of  lenses 128 

80  The  skiascopic  lens  rack  of  Wiirdemann 131 

Si     The    disc    form    of    holding    lenses,    used    by    Crain,    Standart 

and    others 132 

82  The  constructive  principle  of  the  "Jennings"  instrument 133 

83  The  constructive  principle  of  the  "Meyrowitz"  instrument 134 

84  The  constructive  principle  of  the  "Fay"  instrument 134 

85  The  constructive  principle  of  the  instrument  designed  and  used 

by  the  author  in  1892 135 

86  The    constructive   principle    of   the    "Geneva"    instrument   pat- 

ented  about   1898 136 

87  The  constructive  principle  of  the  "Cross"  Retino-Skiameter 138 

88  External  appearance  of  the  "Cross"  instrument 140 

89  Showing  how  to  alter  pupillary  distance 141 

90  Showing  manner  of  using  instrument 142 

91  Showing  instrument  in  dust-proof  case 143 

92  Showing  comparative  size  of  normal  and  magnified  pupil 146 

93  Sample  record  blank 148 

94  Manner  of  filing  records I49 


A  System  of  Ocular  Skiametry 


CHAPTER  I. 

A  Descriptu'e  Name. — Amount  of  Optical  Knowledge 
Necessary  to  Achieve  Skiamf.tric  Success. — The 
Value  of  Bowman's  Discovery. — Difficulties  of  Ocu- 
lar Skiametrv. 

A  DESCRIPTIVE  NAME.  It  is  a  very  doubtful  ques- 
tion whether  any  method  for  examining  an  eye,  in  any  manner 
whatsoever,  can  be  given  as  many  names,  each  with  as  plausi- 
ble a  relevancy  as  those  already  given  to  the  phenomena  of 
light  and  shadow  seen  in  the  pupil  of  an  eye.  These  phe- 
nomena having  been  first  described  by  Sir  William  Paget 
Bowman  some  forty  years  ago,  and  later  known  as  the  ''Sha- 
dow test." 

Since  the  introduction  of  the  blanket  word  "Optometry." 
to  cover  all  measurements  of  an  eye  whether  external,  internal 
or  functional,  the  suffix  "metry"  seems  to  have  been  especially 
assigned  to  duty  in  connection  with  those  ocular  conditions 
wherein  it  is  possible  to  find  a  variance  from  certain  fixed 
standards.  Thus  the  names  Ophthalmometry,  Dioptometry, 
Astigmometry,  Keratometry,  Phorometry,  Strabismometry.  Pu- 
pillometry.  Perioptometry,  and  Visuometry  are  in  use.  Then 
if  the  lines  are  drawn  a  little  finer  there  are  found  such  names 
as  Exophthalmometry,  Ophthalmotometry,  Ophthalmotropo- 
metry, Chromotometry,  and.  in  the  measuring  of  lenses  and 
prisms,  there  are  also  the  words  Phacometry  and  Prisopto- 
metry. 

The  Greek  derivative  "scope"  (sight,  or  examination), 
'.eems  to  have  been  closely  allied  in  the  past  with  shadow-test- 


^^  DESCRIPTIVE    NAME 

ing,  and  the  equivalent  of  such  words  as  shadow-seeing,  retina- 
seeing,  pupil-seeing  and  others  with  visual  reference  have  been 
used. 

The  Greek  words  for  '•seeing"  and   for  "measurin 


are 


sometimes  construed  as  opposites,  but  the  English  word  for 
"measure"  seems  to  be  sharply  defined :  "To  compare  with  a 
fixed  standard."  Now,  in  shadow-testing  this  is  exacth-  what 
an  optometrical  examiner  really  does;  he  measures  an  eye  by 
means  of  reflected  light,  together  with  known  ray-bending  ap- 
pliances called  lenses,  and  compares  his  finding's  with  a  cer- 
tain fixed  standard. 

It  is  common  to  speak  of  measuring  time,  and  it  is  known 
that  light,  shadow  and  sundials  were  formerly  employed  for 
this  purpose.  From  this  to  measuring  passing  shadows  is  not 
a  long  stretch  of  the  imagination,  and  as  an  examiner  must  be 
informed  regarding  the  direction,  speed  and  volume  of  all 
ocular  shadows  in  ascertaining  the  relation  of  the  ray-bending 
power  of  an  eye  to  its  retina,  it  would  seem  that  this  part 
of  a  refractionist'.s  work  comes  nearer  to  being  cye-shadow- 
measuring  than  it  docs  to  anything  else. 

The  plea  that  the  word  ".Skia."'  for  shadow,  should  be 
omitted  from  the  term  used  to  denote  the  shadow  test  does 
not  seem  a  logical  one;  for  from  first  to  last  an  examiner  is 
endeavoring  to  ascertain  the  action  of  the  shadow,  not  of  the 
retina,  by  the  changing  of  lenses  in  front  of  an  eve  for  the 
purpose  of  measuring  the  angle  of  the  emergent  ra\s.  There- 
fore it  follows  that  the  word,  "Retinoscopy,"  though  an  old 
one,  is  not  necessarily  a  good  one,  because  "seeing  the  retina" 
in  this  sense  is  really  not  any  more  descriptive  of  that  which 
is  accomplished  than  would  be  the  same  name  if  applied  to 
that  which  is  now  called  "Ophthalmoscopy." 

The  term  "Skiascopy"  seems  a  better  one  than  that  of 
"Retinoscopy,"    although    seeing-the-shadow    is    not    verv    de- 


NECESSARY   KNOWLEDGE  1 3 

scriptive  either,  unless  it  be  preceded  by  the  general  eye  term 
"ocular,'"'  when  it  could  be  translated  "Eye-shadow-seeing." 

It  has  been  urged  as  an  objection  to  the  use  of  the  word 
"skia"  that  it  might  lead  to  confusion  with  the  pictures,  or 
skiagraphs,  made  by  the  X-rays  of  Roentgen.  In  answer  to 
this  it  may  be  said  that  the  nomenclature  of  optical  science 
is  already  so  complex  that  technical  terms  must  be  given 
sufficient  length  in  order  to  convey  their  proper  meaning, 
otherwise  misunderstanding  and  confusion  between  teacher 
and  student  will  ever  be  on  the  increase.  And  so  it  behooves 
the  ocular-shadow-measurer  and  the  shadowgraph  expert  to 
each  look  to  his  own  descriptions  in  order  that  they  may  meet 
intelligent  requirements. 

The  device  which  reflects  the  light  into  an  eye  and  enables 
an  examiner  to  see  the  shadow  ought  logically  to  be  called 
a  shadow-viewer,  or  skiascope.  And  the  instrument  or 
mechanism  which  permits  of  the  measuring  of  the  angle  under 
which  a  shadow  is  seen  might  correspondingly  be  termed  a 
shadow-measure,  or  skiameter,  while  the  act  or  process  of 
measuring  the  behavior  of  the  shadow  in  an  eye  could  be 
given  the  technical  name  of  ocular  skiametry.  Especially  is 
this  term  applicable,  since,  as  will  be  seen  later  on,  it  is  now 
quite  necessary  to  qualify  and  subqualify  this  term  in  order  to 
represent  gradations  of  meaning  in  the  exact  description  of 
Bowman's  great  discovery,  no  longer  as  a  single  method,  but 
as  a  system  for  examining  and  measuring  the  eye  in  many 
ways. 

AMOUNT  OF  OPTICAL  KNOWLEDGE.  The 
general  optical  principle  of  the  shadow  test  in  its  simplest 
form  is  not  very  complicated.  Taken,  however,  in  connection 
with  its  optometrical  associates  it  represents  as  a  whole  a 
rather  high  order  of  knowledge  regarding  both  physical  and 
phvsiological  optics.     It  also  implies  in  its  application  a  cer- 


14  NECESSARY   KNOWLEDGE 

tain  amount  of  skill  or  dexterity  in  the  manipulation  of  indis- 
])ensable  mechanical  devices,  such  as  the  skiascope  and  ski- 
ameter, no  matter  whether  the  latter  be  a  simple  trial  frame 
with  test  lenses,  or  a  more  elaborate  and  useful  apparatus. 

There  are  two  leading  accomplishments  in  shadow-testing 
in  which  an  examiner  must  be  proficient  before  he  can  achieve 
success  in  this  work.  The  first  of  these  is  the  control  of  the 
reflected  light  and  the  determination  of  the  direction  of  the 
shadow's  motion  under  both  favorable  and  unfavorable  con- 
ditions.  The^ second  lies  in  being  able  to  add  and  subtract 
known  refractive  lens  quantities  and  to  tell  with  precision 
what  their  ray-bending  value  is  at  all  distances  from  an  eye 
under  examination.  To  express  it  tersely  then,  an  examiner 
must  be  able  to  detect  any  action  of  the  shadow  and  to  know 
exactly  what  the  optical  value  of  this  action  is  when  influenced 
by  either  lenses  or  accommodation. 

The  first  of  the  above  requirements  can  be  gained  by  dailv 
practice,  but  the  secorid  requires  considerable  study  and  appli- 
cation, as  it  involves  a  knowledge  of  angles  of  light,  or  ray 
values,  as  well  as  of  refraction,  or  lens  values. 

When  a  patient's  eye  is  considered  as  an  object,  instead  of 
as  a  Subject,  then  its  refractive  condition  must  be  determined 
by  noting  the  behavior  of  the  light  reflected  from  the  retina 
as  it  leaves  the  eye,  and  methods  of  procedure  known  as  "ob- 
jective" must  therefore  be  applied. 

Many  students  of  optics  who  have  confined  their  efforts 
to  a  mastery  of  "subjective"  optometry  find  themselves  quite 
at  sea  when  they  undertake  objective  methods.  And  the 
reason  for  this  usually  lies  in  the  fact  that  they  have  given 
attention  to  the  subject  of  light  as  it  travels  in  one  direction 
only,  namely,  as  it  enters  an  eye. 

One  of  the  foundation  principles  taught  in  optical  text- 
books is  that  light  returns  over  the  same  course  which  it  has 


NECESSARY    K.\()\V[,EDGE  I5 

traveled.  Hence,  if  parallel  rays  of  lij^^ht  are  made  to  pasf 
through  a  convex  lens  they  will  come  to  a  focus  at  the  so 
called  "strength"  of  the  lens.  Invert  this  order,  by  placing- 
a  lighted  candle  at  the  focus  of  the  lens,  and  the  rays  of  ligh» 
will  diverge  until  they  pass  through  the  lens,  after  which 
they  will  be  parallel  again.     See  Figs,  i  and  2. 

Fig.  I 


Sau/(C£ 


parallel  rays  of  light  ueixg  focused. 
Fig.  2. 

__ ^ 


FOCUSED    KAYS    OF    [.IGHT    DEIXG    PARALLELED. 

In  shadow-testing  the  retina  of  an  eye  is  the  apparent 
source  of  light,  although  in  reality  the  retina  is  only  a  noor 
quality  of  mirror  which  reflects  the  light  thrown  into  an 
eye  by  the  skiascope.  This  illumination,  or  reflection,  behaves 
like  a  piece  of  red  flannel,  or  any  other  visible  object  which 
acts  as  a  high  or  low  grade  mirror  according  to  its  quality, 
or  ability,  to  reflect  light,  glass  with  amalgam  backing,  and 
polished  metals,  being  of  the  highest  order,  while  lampblack 
and  black  velvet  are  of  the  lowest. 

The  corelation  of  accommodation  and  convergence  is  an- 
other subject  which  students  of  ocular  skiametry  must  under- 
stand in  order  to  do  their  work  intelligently.  Thus  it  will  be 
seen   that   skiametric   proficiency   involves   a    pretty   thorough 


l6  VALUE    OF    BOWMAN'S    DISCOVERY 

grounding  in  something  more  than  rudimentary  optics.  With 
the  ehmination  of  the  use  of  the  concave  mirror,  however,  and 
by  the  aid  of  modern  apparatus  it  is  now  possible  to  dispense 
with  many  details  which  formerly  resulted  in  the  confusion 
of  beginners.  Still,  notwithstanding  this,  a  student  will  find 
much  that  will  call  forth  his  best  efiforts  before  he  can  feel 
assured  of  the  reliability  of  his  findings. 

It  is  one  thing  to  master  ocular  skiametry  under  regular 
conditions  and  quite  another  to  rightly  differentiate  the  irregu- 
lar and  apply  that  judgment  which  secures  success.  But,  as 
in  other  studies,  the  deeper  the  student  delves  the  more  he 
finds  to  learn,  and  the  easier  do  the  foundation  principles 
become.  The  wise  searcher  after  the  optical  facts  which  in 
the  aggregate  constitute  optical  knowledge  will  first  learn  the^ 
A,  B,  C  of  light  and  lenses,  together  with  the  reduction  and 
transposition  of  the  latter,  and  next  he  will  master  physiological 
optics,  in  addition  to  the  art  of  subjectively  correcting  with 
lenses  any  manifest  conditions  which  may  be  met.  The  stu- 
dent may  then  be  said  to  possess  sufficient  optical  knowledge 
to  begin  the  study  of  ocular  skiametry  with  a  fair  chance  of 
achieving  success. 

THE  VALUE  OF  BOWMAN'S  DISCOVERY. 
Many  inquirers  into  the  merits  of  shadow-testing  seem  to  ^ 
possessed  with  the  idea  that  every  case  which  presents  itseK 
is  capable  of  being  both  easily  and  accurately  refracted  by 
means  of  Bowman's  discovery.  This  expectation  is  as  in- 
consistent with  the  real  facts  as  it  would  be  to  expect  like 
results  from  trial  case  tests  or  any  other  one  optomctrical 
method. 

The  truth  can,  perhaps,  be  fairly  expressed  by  saying  that 
shadow-testing  bears  to  trial  case  testing  much  the  same  rela- 
tion as  the  addition  of  a  column  of  figures  from  the  top  bears 
to  its  addition  from  the  bottom. 


VALUE    OF    BOWMAN  S    DISCOVERY  I7 

Skiascopy  will  uncover  at  a  single  sitting  optical  condi- 
tions which  it  would  be  quite  impossible  for  ordinary  trial 
case  tests  to  do.  On  the  other  hand,  the  latter  will  show 
visual  conditions  of  which  the  former  can  tell  nothing. 
\'iewed  again  from  a  similar  position  we  find  that  the  two 
general  methods  for  estimating  ocular  errors  of  refraction, 
known  by  the  terms  "objective"  and  "subjective,"  are  like 
seeing  for  one's  self  and  taking  the  testimony  of  others. 
Usually  either  method  is  fairly  reliable  in  ordinary  cases,  but 
in  extraordinary  ones — the  kind  that  make  and  break  reputa- 
tions— the  evidence  can  be  none  too  corroborative. 

So  we  find  skiascopic  and  trial  case-testing  to  be  inter- 
dependent, both  systems  having  their  weak  and  strong  points 
and  one  aiding  in  the  judgment  requisite  for  the  successful 
application  of  the  other,  skiascopy  coming  first  because  it 
is  the  great  refractive  pilot,  or  pathfinder,  and  because,  too. 
there  are  many  conditions  other  than  errors  of  refraction 
that  are  shown  up  by  its  use,  and  which,  if  it  were  not  for 
this  early  use,  might  needlessly  prolong  an  otherwise  short 
examination. 

It  is  hoped,  therefore,  that  this  point  is  made  clear  regard- 
ing the  value  of  shadow-testing.  It  is  deemed  of  no  more,  nor 
less,  value  than  the  trial  case  test,  and  that  neither  one  is 
infallible,  and  that  both  are  absolutely  essential  in  all  prime 
cases,  whether  the  results  obtained  by  either  coincide  with 
those  of  the  other  or  not,  for  this  very  lack  of  coincidence 
is  often  the  key  which  enables  a  trained  judgment  to  solve 
a  refractive  riddle. 

By  basing  their  judgment  upon  the  principle  that  the 
proof  of  the  pudding  lies  in  interviewing  him  who  has  chewed 
the  string,  some  credulous  inquirers  have  been  led  to  estimate 
the  merits  of  shadow-testing  by  taking  the  testimony  of  those 
who  have  falsely  pretended  to  possess  a  thorough  knowledge 


l8  VALUE    OF    bowman's    DISCOVERY 

of  it,  and  this  unreliable  information  has  led  them  to  believe 
that  if  skiametry  is  faulty  in  some  hands  it  must  be  faulty 
in  all.  Whereas  the  reverse  reasoning  would  in  all  probability 
be  productive  of  better  results,  for  that  which  one  can  achieve 
by  study  and  practice  it  is  (|uite  possible  for  others  to  achieve 
by  equal  application  and  efifort,  and  sometimes  by  even  less 
where  assistance  is  given  by  skilled  tutors. 

For  nearly  four  decades  the  ablest  optometrical  researchers 
have  striven  their  utmost  to  find  a  better  objective  means  than 
skiametry  for  determining  the  optical  condition  of  eyes,  but 
so  far  without  avail.  And  judging  from  the  present  advanced 
knowledge  regarding  optics  and  optometr_\-  it  is  pretty  safe 
to  say  that  the  shadow  test  is  here  to  stay,  for  a  long  time 
at  least,  and  that  those  whose  duty  it  is  to  adapt  glasses  to 
the  eyes  of  others  will  find  their  work  more  reliable  and 
much  easier  if  they  take  the  time  to  thoroughly  master  this 
valuable  means  for  ascertaining  ocular  refractive  conditions 
in  a  manner  independent  of  the  patient's  intelligence. 

Xow  this  phrase,  "independent  of  the  patient's  intelligence," 
may  prove  somewhat  misleading,  since  even  those  who  are 
experienced  in  skiametric  work  find  many  cases  in  which  the 
results  obtained  are  very  unsatisfactory  indeed.  Yet,  wlion 
an  examiner  measures  a  case  by  skiametry  and  notes  an  error 
of  refraction  which  later  on  is  confirmed  by  the  trial  case 
test,  he  feels  that  he  has  received  advanced  information  of  a 
truly  "independent"  character,  upon  which  he  can  rely  with 
greater  confidence  than  if  this  information  had  been  denied 
him.  r)n  the  other  hand,  if  the  trial  case  test  does  not  con- 
firm the  mirror  findings,  then  the  mirror  is  employed  again 
to  confirm  the  trial  case  findings.  The  subjective  is  used  to 
check  the  objective,  and  then  the  objective  again  to  corroborate 
the  subjective. 


DIFFICULTI'ES    OF    SKIAMFTKV  I9 

DIFFICULTIES.  The  stumblinc;^  blocks  in  ocular  ski- 
ametry  are  not  few,  and  the_\  seem  to  grow  apace  as 
the  system  becomes  perfected  in  its  many  details.  The 
first  great  obstacle  which  usually  presents  itself  is  place,  or 
examination  room.  The  medical  refractionist  who  takes  up 
this  work  may  already  be  provided  with  a  regulation  dark- 
room for  his  ophthalmoscopic  work,  and  in  this  room  he 
attcmi)ts  to  ])ractice  successful  skiametry.  The  conditions 
being  poor,  perhaps,  he  gets  poor  results  and  abandons  the 
work  in  the  belief  that  shadow-testing  is  sadly  overrated. 

The  non-medical  refractionist  may  possibly  go  to  the  other 
extreme.  The  specious  plea  that  a  dark-room  is  unnecessary 
is  listened  to,  then,  not  being  able  to  use  a  cycloplegic,  and 
having  a  knowledge  only  of  the  static  method,  as  it  is  now 
called,  he  wonders  why  his  skiametric  work  varies  so  with 
his  trial  case  findings.  And  then,  too.  because  he  possesses 
a  sufficient  degree  of  skill  to  feel  comparatively  sure  of  the 
action  of  the  shadow-  in  an  occasional  case,  he  either  blames 
the  system  or  virtually  condemns  it  by  faint  praise. 

Next  to  a  poorly-arranged  examination  room  in  point  of  dis- 
couragement with  shadow-testing  comes  an  inadequate  source 
of  illumination.  Almost  any  lamp,  whether  electric,  gas.  or 
oil.  looks  to  a  novice  as  though  it  ought  to  prove  of  sufficient 
intensity  to  determine  a  shadow's  action,  because  when  the 
light  is  reflected  into  a  naked  eye  the  fundus  reflex  seems 
fairly  bright,  but  by  placing  a  lens  or  two  in  front  of  this 
eye,  or  by  permitting  the  patient  to  look  in  an  unfavorable 
direction,  so  pronounced  a  diminution  will  often  be  produced 
in  the  definition  of  the  shadow  as  to  render  accurate  work 
impossible. 

Even  where  the  source  of  illumination  is  up  to  the  standard 
of  forty-candle  power,  or  more,  the  experienced  examiner 
will  meet  with  cases  where  the  deeply  pigmented  retina  gives 


20  DIFFICULTIES    OF    SKIAMETRY 

back  so  poor  a  reflection  that  only  the  greatest  care  and  skill 
can  determine  the  action  of  the  shadow. 

There  is  a  vast  difference  in  general  illuminating  power 
between  a  flame  that  is  three-quarters  hooded  and  one  that  is 
not,  and  a  dark-room  light  to  be  satisfactory  must  not  be  too 
large.  Its  apparent  intensity  should  resemble  the  "Glory" 
'hole  of  a  furnace,  and  then,  if  this  should  prove  too  bright 
for  an  occasional  supersensitive  eye,  it  can  always  be  dimin- 
ished by  moving  the  patient  farther  away  from  it. 

The  law  that  light  decreases  in  proportion  to  the  square 
of  the  distance  at  which  it  is  used  enables  a  light  intensity 
equal  to  sixty-candle  power  at  two  feet  away  to  be  decreased 
to  fifteen-candle  power  by  withdrawing  to  a  distance  of  four 
feet.  Thus  it  will  be  seen  that,  with  a  powerful  source  of 
illumination,  an  examiner  can  readily  obtain  almost  any  candle 
power  he  desires. 

Just  how  great  a  candle  power  the  human  eye  can  bear 
without  doing  it  injury  varies  undoubtedly  with  individuals 
and  the  duration  of  the  exposure.  A  hundred-candle  power 
lamp,  hooded  so  that  only  a  limited  portion  of  its  general 
radiant  energy  is  available,  could  probably  be  comfortably 
borne  by  the  average  eye  under  ordinary  skiametric  condi- 
tions for  several  minutes  at  a  time,  whereas  the  length  of  time 
for  a  proper  measurement  is  only  a  matter  of  seconds.  Too 
bright  a  light,  therefore,  need  not  be  feared. 

In  using  a  bright  light,  however,  there  is  one  thing  an 
examiner  should  always  remember,  as  it  can  properly  be  classi- 
fied among  the  stumbling  blocks  to  be  avoided,  and  that  is 
to  never  allow  himself  to  look  directh'^at  the  sour  eg  of  his 
illumination  prior  to  or  during  an  examination.  The  reason 
is  that  the  sensitiveness  of  his  own  retina  is  such  as  to  retain 
the  impressions  made  by  a  bright  light  to  such  a  degree  that 
duller  objects  cannot  be  clearly  seen  for  several  minutes  after 


DimCULTIES    OF    SKIAMETRY  21 

looking  at  a  bright  flame.  And  as  a  consequence,  if  an  ex- 
aminer" permits  himself  to  look  directly  at  his  lamp  for  an 
instant  or  two  and  then  tries  to  use  his  mirror  he  will  find  it 
very  difficult  to  detect  the  dull  outline  of  the  shadow  or  note 
its  action.  If  an  inspection  of  the  lamp  is  necessary  to  deter- 
mine its  condition  or  distance  away  it  is  a  wise  examiner  who 
will  derive  his  information  by  looking  a  few  inches  to  one 
side  of  the  flame  and  not  directly  at  it. 

Another  stumbling  block  in  the  road  to  skiametrical  suc- 
cess lies  in  corroded,  soiled  and  dusty  mirrors,  especially  at 
or  about  the  so-called  "peephole"  of  the  skiascope.  Every 
examiner  should  possess  at  least  two  or  more  mirrors,  so 
that  when  one  gets  out  of  order  it  can  be  sent  to  the  factory 
for  resilvering.  Mirrors,  to  give  the  best  service,  must  not 
be  of  too  great  a  diameter,  nor  must  their  peepholes  be  large 
or  bored  through  the  glass.  Consequently  these  holes,  which 
are  really  not  holes  in  the  strict  sense  of  the  word,  but 
round  spots  of  clear  glass  made  by  scraping  the  silver  off  the 
mirror,  must  be  kept  immaculately  clean,  so  as  to  prevent 
particles  of  dust  from  interfering  with  the  free  passage  of 
light,  or  else  the  shadow's  action  will  be  perceptibly  dimmed. 

One  of  the  underlying  causes  of  the  success  achieved  in 
many  branches  of  modern  science  is  undoubtedly  due  to  ultra- 
cleanliness  and  attention  to  details,  and  so  it  will  be  with 
advanced  optometry  as  regards  details.  For  dealings  with 
imaginary  quantities  of  elastic  ether,  as  light  is  termed,  call 
for  extreme  care  on  the  part  of  the  examiner,  if  he  is  of  the 
kind  that  are  satisfied  with  nothing  short  of  the  highest  attain- 
ment. 

A  thorough  knowledge  of  lenses  is  still  another  important 
factor.  There  is  probably  no  one  study  connected  with  a 
skiascopist's  educational  equipment  which  demands  a  more 
perfect  mastery  than  does  that  of  the  reducing,  transposing 


22  DIFI-ICULTIliS    OF    SKIA^n-:TRY 

and  combining  of  lens  values.  In  his  desire  to  attain  perfec- 
tion on  the  practical  side  of  adapting-  glasses  to  the  eyes  ,of 
others  the  student  is  apt  to  pass  hurriedly  by  the  Iry  under- 
lying optical  principles  upon  which  lenses  are  based  and,  as  a 
consequence,  after  his  first  few  years  of  "success  in  every  case" 
his  desire  to  climb  higher  is  interfered  with  through  his  lack 
of  knowledge  of  that  which  he  ought  to  have  learned  at  the 
beginning  of  his  optical  career. 

In  ocular  skiametry  a  simple  stumbling  block  to  many 
students  is  their  inability  to  tell  what  the  true  refraction  of 
their  patient's  eye  is,  when  it  takes  an  ordinary  lens  quantity 
of,  say,  one  diopter  to  reverse  the  shadow  in  one  meridian, 
while  only  a  half-diopter  is  required  to  reverse  it  in  the 
meridian  at  right  angles  to  this.  Especially  is  this  true 
where  the  axes  happen  to  be  oblique,  or  where  one  lens  is 
plus  and  the  other  niiiius.. 

Now  this  is  all  wrong,  for  if  an  optometrician  ought  to 
know  anything  he  oughtjo  know  all  about  lenses — how  they 
are  made,  what  index  of  refraction  and  spherical  ab^^rration 
mean,  wherein  cylindrical,  spherical  and  elipsoidal  curv-ntures 
dififer  and,  above  all,  the  very  best  way  to  combine  lens  quanti- 
ties in  order  to  produce  the  highest  degree  of  central  and 
peripheral  vision,  together  with  angle  and  decentration  foi 
the  relief  of  strain  and  the  production  of  comfort. 

A  thorough  knowledge  of  light  and  lenses  is  the  great 
key  to  the  unlocking  of  skiametric  success.  After  a  student 
has  learned  all  about  the  j^hys'ca]  side  of  refraction  work  the 
physiological  becomes  easier,  just  as  a  mathematician  who  has 
skipped  decimal  fractions  finds  his  higher  work  puzzling,  but 
if  he  has  covered  his  ground  carefully  then  his  advancement 
is  less  difficult. 

A  very  few  sittings  will  give  a  student  the  mastery  of  a 
skiascopic  mirror  and  enable  him  to  tell  the  movement  of  the 


DIIFRLLTIKS    OF    SKIAMliTUV  2T, 

pupillary  shadows,  but  how  to  control  these  shadows  by  the 
aid  of  lenses  combined  with  accommodation,  and  to  know 
the  real  optical  value  of  the  refractive  power  used,  is  where 
one  of  the  greatest  of  the  stumbling  blocks  in  shadow-testing 
lurks,  so  that  those  who  become  discouraged  in  their  skia- 
metric  efforts  must  not  blame  this  grand  test,  but  look,  rather, 
to  their  own  weakness  in  their  lack  of  adequate  optical  knowl- 
edge and  skill. 

There  is  one  more  point  to  which  attention  should  be 
directed,  although  it  can  hardly  be  called  a  "stumbling  block," 
even  though  in  some  cases  it  seems  to  act  as  such.  It  pertains 
to  an  examiner's  own  vision,  which  should  be  such  as  to  enable 
him  to  see  with  definition  a  moderately  pale  shadow  on  a  pink 
background  at  a  distance  of  at  least  forty  inches.  Therefore 
all  examiners  having  myopia  must  have  their  own  refractive 
errors  corrected  to  within  less  than  one  diopter  before  they 
can  do  successful  skiametric  work.  All  hypermetropes.  on  the 
other  hand,  whose  amplitude  of  accommodation  at  thirteen 
inches  is  less  than  their  error  of  refraction  must  also  wear 
their  correcting  lenses  before  they  can  make  satisfactory 
shadow-tests.  It  would  seem  that  the  peephole  in  a  skiascope 
ought  to  act  as  a  pinhole-test  and  give  any  examiner  good 
vision,  no  matter  what  his  refractive  error  might  be,  but 
experience,  that  great  teacher,  rules  it  otherwise  in  those  cases 
where  high-class  results  are  sought  for. 


CHAPTER     II. 

Adequate  and  Inadequate  Examination  Rooms. — Illu- 
mination:— Its  Size,  Source  and  Control. — The 
Plane  Mirror: — Its  Construction  and  How  to 
Handle  It. — Good  and  Bad  Schematic  Eyes,  and  How 
to  Correct  Them  : — A  Short  Method  for  the  Reduc- 
tion AND  Transposition  of  Lens  \^'\lues. 

EXAMINATION  ROOMS.  To  describe  an  ideal  exam- 
ination room  is  quite  a  different  matter  from  attempting  the 
description  of  an  adequate  apartment,  or  compartment,  that 
will  serve  fairly  well  as  a  place  in  which  to  practice  ocular 
skiametry. 

The  complaint,  or  excuse,  that  is  frequently  heard  from 
many  who  attempt  to  do  refraction  work  without  having  suit- 
able place  and  apparatus  is  that  they  lack  the  requisite  space, 
wh'^reas,  an  examination  of  the  store  or  office  by  one  experi- 
enced as  to  requirements  might  lead  to  the  discovery  of  quite 
a  number  of  places  which  could  be  rendered  available  by  the 
use  of  properly  strung  wires  for  hanging  light-proof  curtains, 
which  could  also  be  made  decorative  in  appearance.  Then, 
too,  many  are  persuaded  that  an  examination  room  must  be  of 
Egyptian  darkness,  and  without  ventilation,  where  both  exam- 
iner and  patient  will  be  very  uncomfortable,  especially  during 
warm  weather. 

Now,  this  is  a  wrong  conception.  An  adequate  examina- 
tion room  needs  to  be  of  only  semi-darkness.     In  fact,  if  it 


EXAMINATION    ROOMS  2$ 

is  light  enough  to  make  the  head-lines  of  an  ordinary  news- 
paper just  discernible  at  midday,  it  will  be  found  quite  dark 
enough  for  all  optometrical  purposes.  The  "Mahomet  and 
the  mountain''  principle  can  be  made  use  of  by  increasing  the 
intensity  of  the  light  source,  instead  of  making  the  room  appear 
as  dark  and  gloomy  as  the  interior  of  a  hearse. 

The  space  occupied  need  not  be  very  wide.  The  length 
of  room,  however,  ought  to  be  at  least  twenty  feet,  but  where 
this  distance  cannot  be  obtained  a  length  of  ten  feet  can  be 
made  to  appear  as  twenty  by  the  use  of  an  ordinary  wall 
mirror,  test  cards  with  reversed  letters  being  placed  over  the 
head  of  the  patient  in  a  position  Avhere  their  reflection  can 
readily  be  seen  in  the  mirror. 

The  ventilation  should  be  perfect,  and  the  space  ought  to 
contain  several  chairs  for  the  use  of  those  who  accompany  the 
patient,  while  the  surroundings  should  be  made  as  cheerful 
as  they  can  be  made  by  means  of  rugs,  pictures,  etc. 

Both  objective  and  subjective  tests  should  be  used  without 
having  to  move  the  patient  from  the  chair  in  which  he  is  first 
seated,  and,  where  possible,  the  better  way  is  to  bring  all 
instruments  and  devices  to  the  patient  rather  than  require  the 
patient  to  go  to  them. 

First  impressions  are  said  to  be  lasting,  so  that  if  patients 
are  shown  into  apartments  that  look  as  though  they  were 
intended  for  optometrical  purposes  their  confidence  is  more 
than  half  won. 

Doing  ocular  refraction  w^ork  over  a  showcase  is  about  as 
inappropriate  and  non-professional-looking  as  it  would  be  to 
practice  dentistry  in  a  like  manner.  There  is  a  fitness  of  things, 
and  those  whose  practice  is  in  accord  with  this  "fitness"  gen- 
erally have  occasion  to  feel  satisfied  with  the  results  obtained 
from  having  given  attention  to  the  details  of  environment. 
Indeed,  it  has  already  been  said  by  many  that,  next  to  profes- 


26  ILLUMINATION 

sional  knowledge  and  skill,  a  well  appointed  examination  room 
is  the  very  best  kind  of  an  advertisement  that  a  refractionist 
can  possibly  have. 

ILLUMINATION.  One  of  the  "stumbling  blocks"  re- 
ferred to  in  the  preceding  chapter,  to  which  the  reader's  atten- 
tion has  already  been  called,  is  the  importance  of  using  a  proper 
source  of  illumination  in  order  to  succeed  in  shadow  testing. 
Therefore  it  will  be  to  the  purpose  here  to  describe,  with  the 
assistance  of  drawings  and  photo-reproductions,  some  of  the 
various  lamps  employed  for  this  purpose  and  to  offer  some 
criticisms  regarding  the  advantages  and  disadvantages  conse- 
quent upon  their  use. 

Practising  skiametry  by  means  of  a  model,  or  so-called 
"schematic,"  eye  is  very  easy  in  comparison  to  practising  it 
upon  a  living  eye,  for  with  a  model  eye  almost  any  kind  of 
light  will  answer,  but  not  so  with  the  living  organ,  where,  as 
before  stated,  the  light  source  should  resemble  the  glow  ema- 
nating from  the  "Glory"  hole  of  a  furnace. 

Oil  lamps,  even  of  the  "Rochester"  and  "Success"  burner 
types,  while  they  are  magnificent  for  general  lighting  purposes, 
fall  below  the  standard  of  efficiency  when  employed  for  general 
skiametric  uses.  The  draught  in  almost  all  lamps  is  an 
important  factor,  and  as  metal  chimneys  cannot  be  easily  made 
to  take  the  place  of  glass  ones,  on  account  of  the  transparent 
aperture  which  is  required  opposite  the  flame,  it  has  been  found 
necessary  to  either  line  or  cover  all  glass  chimneys  with  an 
opaque  substance  which  extreme  heat  cannot  affect.  Fig.  3 
illustrates  one  of  the  best  types,  perhaps,  of  any  of  the  oil 
lamps,  but  even  where  the  chimney  is  lined  with  a  white  pig- 
ment it  still  falls  short  of  giving  satisfaction  as  an  efficient 
illuminator,  although  the  maker  may  claim  that  his  lamp  has  a 
general  efficiency  of  over  one  hundred-candle  power. 


ILLUMINATION 


27 


Ihe  reason  for  the  shortcomings  of  almost  all  oil  lamps 
using  a  so-called  "flame  spreader"  is  due,  no  doubt,  to  the 
fact  that  the  flame,  though  large,  is  apt  to  be  very  low,  and 

Fig.  3. 


A     SUCCESS     IU:RM:K  OIL  LAMl' 


when  its  general  intensity  is  hooded  down  to  the  size  of  a  five 
cent  piece,  which  is  necessary  in  order  to  obtain  good  results 
with  a  plane  mirror,  the  flame  is  found  of  insufficient  bright- 
ness to  meet  an  examiner's  needs. 


28 


ILLUMINATION 


This  reasoning  also  applies  to  nearly  all  kerosene  lamps  as 
now  used.  Perhaps  some  day  some  inventive  mind  will  devise 
a  condensing  reflector  which  will  permit  of  hooding  the  light 
down  to  a  small  aperture  and  at  the  same  time  obtain  the 
requisite  intensity  of  illumination,  but  as  yet  this  has  not  been 
achieved. 

Next  to  oil  comes  gas.  And  here  the  field  of  illumination 
broadens,  thanks  to  the  inventors  of  the  "Argand"  and  "Wels- 
bach"  burners,  and  to  the  gas-generating  qualities  of  naphtha 

Fig.  4. 


AN       ARGAND       BURNER    GAS    L.\MP 


and  of  acetylene  products.  Ranking  above  oil  burners  comes 
the  Argand  gas  lamp,  which  also  requires  a  draught  to  make 
it  burn  properly,  hence  a  glass  chimney  is  necessary  for  its  use. 
Fig.  4  illustrates  this  type  of  lamp. 

And  while  this  is  vastly  superior  to  oil  lamps  in  general,  it 
is  still  somewhat  below  the  required  standard  of  efficiency, 
even  when  working  at  its  very  best.  The  flame  has  a  yellowish 
white  appearance,  and  seems,  like  the  flame  of  an  oil  lamp,  to 


ILLUMINATION  29 

lack  the  illuminating  energ-y  necessary  to  meet  modern  skia- 
metric  needs,  although  in  favorable  cases  fair  work  can  be 
done  by  its  aid. 

Acetylene  lamps  represent  another  style  of  gas  burners 
which  in  point  of  intensity  of  illumination  can  probably  hold 
their  own  against  all  competitors.  The  style  shown  in  Fig,  5 
represents  a  portable  type. 

Fig.  5. 


AN   ACETYLENE  GAS   LAMP 

This  lamp  is  similar  in  appearance  to  an  ordinary  table 
lamp,  with  the  exception  that  it  has  an  asbestos-lined  metal 
chimney.  It  is  commercially  named  an  "Electrolite  gas  lamp," 
and  uses  specially  pulverized  calcium  carbide.  Its  use  is 
endorsed  by  the  board  of  fire  underwriters,  and  it  is  easy  to 
care   for.     The  light   it   gives  is   over   fifty-candle  power  in 


30 


ILLUMINATION 


intensity,   while  the  expense  of  maintaining  it   is  only  about 
one  cent  an  hour. 

Skiascopists  are  certainly  to  be  congratulated  on  the  inven- 
tion of  this  lamp,  especially  those  who  are  compelled  to  do 
work  away  from  their  properly  appointed  examination  rooms, 
for  as  a  portable  lamp  it  is  very  satisfactory. 

Fig.  6. 


*5 


WELSBACH       GAS    LAMP 


In  point  of  brilliancy,  reliability,  cost  of  maintenance  and 
ease  of  adjustment,  however,  probably  no  lamp  is  superior  to 
the  Welsbach  type,  especially  with  a  non-breakable,  asbestos- 
lined  chimney,  as  shown  in  Fig.  6. 


ILLUMINATION 


31 


This  burner  is  of  the  "Incandescent"  kind,  wherein  chimney 
draught  is  not  a  factor,  so  that  a  glass  chimney  is  not  really 
needed,  although,  owing  to  its  low  cost  and  fine  appearance 
some  examiners  prefer  a  glass  to  a  metal  one.  Figs.  7  and  8 
show  two  patterns  of  glass  chimneys  which  can  be  used  on  the 
same  style  of  burner. 

Fig.  7. 


A  "welsbach"  gas  lamp  with  large  asrestos-lined  "cross' 

CHIMNEY. 


The  Fig.  7  chimney  being  farther  away  from  the  mantle, 
or  flame,  the  glass  does  not  get  as  hot  as  it  does  in  the  pattern 
Fig.  8,  but  the  smaller  the  chimney  the  more  convenient  is 
the  adjustment  of  the  light,  the  source  of  illumination  beingf 


32 


ILLUMINATION 


nearer  to  tlie  surface  of  the  screen  ;  therefore  the  Fig.  8  style 
is  to  be  preferred. 

Fig.  8. 


A     SMALL    ASBESTOS-LINED       CROSS       CHIMNEY     FOR     EITHER     A 
"wELSBACH"    or   an    "aRGAND"    LAMP. 


The  Welsbach  lamp  gives  an  ideal  light  for  shadow-testing, 
its  flame  is  bluish  white,  and  its  intensity  is  ample  when  its 
mantle  is  properly  heated  to  incandescence.  The  one  unfav- 
orable criticism  which  can  be  made  regarding  it  is  on  the 
fragility  of  its  mantles,  which  are  very  easily  injured  or 
destroyed.  But  fortunately  these  mantles  are  inexpensive  and 
are  not  difficult  of  adjustment. 

In  connection  with  naphtha  or  gasolene,  the  Welsbach 
burner  can  also  be  used,  and  while  the  light  obtained  is  not 


ili.i;mination 


33 


as  satisfactory  as  where  so-called  "City"  gas  is  employed,  yet 
the  results  obtained  are  vastly  superior  to  the  use  of  oil. 
Fig.  9  shows  the  well-known  student  lamp  form  in  which  these 
lamps  are  usually  manufactured. 

Fig.  9. 


A  GASOLENE   "STUDENT 

They  make  an  excellent  portable  light  when  care  is  used  in 
adjusting  them.  The  general  use  of  gasolene  has  its  disad- 
vantages, however,  resulting  from  its  explosive  qualities. 
This  is  a  question  on  which  the  makers  of  gasolene  lamps  and 
the  fire  insurance  companies  are  not  at  all  in  accord.  But  as 
efficient  illuminators  for  skiametric  uses  gasolene  lamps,  using 
the  Wclsbach  type  of  burner,  will  be  found  fairly  satisfactory, 
provided  they  are  always  kept  in  perfect  order. 


34  ILLUMINATION 

Electric  lamps  of  the  general  house-lighting-  variety,  with 
long  carbon  filaments,  are,  perhaps,  among  the  most  unsatis- 
factory of  all  illuminators  for  use  in  connection  with  ocular 
skiametry,  no  matter  whether  the  glass  bulbs  are  of  the  "clear" 
or  of  the  "frosted"  variety. 

An  electric  lamp,  to  be  of  service  in  this  work  must  have 
its  filament  in  compact  form,  similar  in  shape  to  the  coils  of  a 
watch  hairspring.  Then  the  light  energy  of  the  carbon  wires 
when  heated  to  incandescence  can  be  concentrated,  and  the 
results  obtained  can  be  made  to  equal  the  Welsbach  type  of 
lamp.  Fig.  lo  shows  an  electric  lamp  filament  of  the  spiral 
kind  referred  to.  This  lamp  needs  to  be  handled  with  great 
care,  since,  owing  to  the  size  and  brittle  character  of  its  fila- 
ment, it  becomes  easily  broken  l)y  a  sudden  jar  or  through 
rough  handling,  as  in  the  mail,  or  when  shipped  by  express. 

Fig.  io. 


SnOWIXG  SPIRAL  FILAMENT  FOR  ELECTRIC  LAMP. 

Fig.  II  shows  this  same  lamp  with  its  glass  bulb  coated 
with  a  thick  asbestos  pigment,  leaving  a  one-inch  aperture  in 
its  side  through  which  the  light  can  emerge. 

This  lamp  is  rated  at  fifty-candle  power  and  gives  a  mag- 
nificent reddish  white  light.  The  white  hot  filament,  or  carbon 
wires,  generate  considerable  heat,  which,  of  course,  will  melt 
the  rubber,  or  composition,  socket  handle  if  the  lamp  happens 
to  be  used  upside  down.     The  heat  from  one  of  these  lamps 


ILLU.MIXATIOX 


35 


has  also  been  known  to  char  th^  curtain  hangings  of  a  window 
with  which  it  came  in  contact.  But  used  as  it  is  intended  to 
be  used,  this  style  of  lamp  certainly  supplies  an  adequate 
illumination  for  any  kind  of  examination  room,  whether  light 
or  dark,  or  where  gas  cannot  be  obtained,  such  as  in  modern 
office  buildings,  etc. 

Fi(,.  II. 


THE      CROSS      ASBESTOS  COVERED  ELECTRIC   LAMP. 


The  cost  of  maintenance  of  an  electric  lamp  is  somewhat 
higher  than  that  of  a  Welsbach  gas  lamp,  and  its  durability  is 
not  as  great.  It  requires  careful  adaptation  to  the  voltage  of 
local  electric  currents.  Any  increase  in  power  even  of  only  a 
few  volts,  such  as  frequently  occurs  in  cities  where  the  current 
is  intensified  at  sundown,   serves  to  shorten   the  life  of  the 


36 


ILLUMINATION 


lamp  very  materially.  These  uncertainties  of  current  can  be 
controlled  by  what  are  called  resistance  attachments,  or 
rheostats,  if  an  examiner  cares  to  incur  the  expense.  All  high 
candle  power  electric  lamps  are  sensitive  to  abuse,  but  the 
superior  light  given  by  them  more  than  compensates  for  the 
trouble  and  expenditure  their  use  entails. 

Fig.  12. 


THE     DE  7ENG      LUMINOUS   RF.TINOSCOPE. 


Another  form  of  electric  illumination,  which  has  the  advan- 
tage of  portability,  is  the  combined  lamp  and  mirror  known  as 
the  "Dc  Zcng  luminous  rclinoscopc."  Fig.  12  illustrates  its 
general  appearance. 


ILLUMINATIOX 


37 


The  lamp  used  is  of  only  one  or  two-candle  power  and  is 
located  behind,  and  close  to,  a  strong  convex  lens,  which 
serves  to  parallel  the  light  radiation  and  thereby  avoid  any 
waste  due  to  this  cause.  It  obtains  its  electrical  energy  by 
means  of  either  a  storage  battery  or  from  a  general  house 
current  passed  through  a  resistance  attachment,  but  like  other 
electric  lamps  it,  too,  is  sensitive  to  abuse. 

The  light  it  gives  is  a  fairly  intense  one,  which,  even  though 
the  colors  of  the  spectrum  are  quite  pronounced,  enables  good 
work  to  be  accomplished  in  the  static  method  by  those  who 
are  acquainted  with  its  peculiarities.  In  the  dynamic  method, 
however,  it  fails  to  illuminate  the  examiner's  fixation  cards,  as 
the  regular  lamps  do,  and  therefore  an  additional  light  for  this 
purpose  is  required.  As  a  portable  light  it  is  compact  and 
easy  of  transportation  and  serves  the  purpose  of  being  useful 
as  an  ophthalmoscope  as  well. 

Fig.  13  . 


B 


B 


WALL  BRACKET  FOR  GAS  OR  ELECTRIC  LAMP. 

A  point  to  bear  in  mind  in  connection  with  adequate 
illumination  in  skiametric  work  is  to  have  whatever  lamp  may 
be  used  so  arranged  that  its  adjustments,  forward  and  back- 


3&  MIRRORS 

ward,  to  and  from  an  examiner's  own  eye,  is  readily  obtainable. 
The  adjustment  as  to  heij^ht  is  not  so  important  as  long  as  the 
light  is  about  level  with  the  patient's  head  and  is  situated  from 
six  to  twelve  inches  to  the  patient's  right.  The  alterable  dis- 
tance forward  and  backward  is.  however,  quite  essential,  as  it 
enables  the  intensity  of  illumination  to  be  controlled  by  an 
examiner  as  his  case  demands.  A  very  simf)le  way  to  arrange 
for  this  is  to  use  a  two  or  thrcc-arm  wall  bracket  similar  to 
that  in  Fig.  13. 

This  bracket  can  also  be  used  to  carry  an  electric  bulb,  and 
thus  give  an  examiner  a  double  system  of  illumination  which, 
in  emergency,  may  prove  of  very  great  value  in  preventing  a 
break-down. 

In  summing  up  the  question  of  illumination,  perhaps  the 
expression  from  the  pen  of  a  western  specialist  will  serve  to 
state  the  case  fairly  well.  He  recently  wrote,  "I  fully  realize 
that  proper  illumination  is  the  foundation  of  success  in  skias- 
copy." And  it  may  be  added  that  this  opinion  is  shared  by 
many  others  who  have  had  experience. 

THE  PLAMi  MIRROR.  The  confusion  following 
the  use  of  two  forms  of  skiascopes,  such  as  those  having  plane 
and  those  having  concave  mirrors,  has  led  to  the  virtual 
abandonment  of  the  latter  form  by  many  of  the  ablest  skias- 
copists  of  the  country.  There  are  possibly  some  conditions 
under  which  a  concave  reflector  might  give  an  examiner 
better  service  than  a  plane  one  would,  but  these  are  rare,  and 
for  general  all-round  skiametric  i)urposes  the  plane  mirror  is 
to  be  greatly  preferred.  All  mirrors  should  be  as  brilliantly 
silvererl  as  possible,  and  the  reflections  from  them  ought  to  be 
perfectly  round  and  free  from  distortion. 

Regarding  the  size  or  working  part  of  a  mirror,  this  can  be 
easily  determined  by  holding  it  at  the  maximum  distance  at 
which  it  is  to  be  used,  and  covering  its  periphery  with  washer- 


MIRRORS  39 

like  pieces  of  paper.  The  size  of  the  reflecting  surface  neces- 
sary to  produce  the  best  results  while  in  actual  service  can 
then  be  noted.  This  will  usually  be  found  to  represent  an  area 
of  about  three-quarters  of  an  inch  in  diameter. 

The  central  aperture,  or  peep-hole,  should  be  as  small  as 
possible  and  yet  permit  of  acute  vision  on  the  part  of  the 
examiner.  A  diameter  of  one  or  two  millimeters  is  generally 
sufficient  for  the  purpose.  Having  the  handle  at  least  six 
inches  long  will  also  be  found  advantageous. 

If  the  metal  disc  holding  the  mirror  is  concaved  at  its 
back,  as  shown  in  Fig.  14,  it  will  be  of  assistance  in  keeping 
the  peep-hole  free  from  dust  and  dirt.  A  frequent  twist  of 
the  skiascope  just  before  it  is  used,  and  while  its  front  and 
back  are  covered  with  a  handkerchief  held  between  the  thumb 
and  forefinger  of  the  operator's  hand,  is  all  that  is  necessary 
in  order  to  keep  it  quite  clean. 

Fig.  14. 

section  of  the  "cross"  mirror  showing  concave  b.\ck. 

As  will  be  seen  in  subsequent  chapters,  the  use  of  a  card 
for  fixing  the  vision  of  the  patient  at  the  same  distance  away 
as  that  at  which  the  mirror  is  operated  renders  some  means 
for  attaching  a  card  to  the  skiascope  almost  a  necessity.  Fig. 
15  shows  a  device  to  which  the  name  "Bracket  Mirror"  has 
been  given. 

The  arrangement  of  the  bracket  attachment  to  this  mirror 
is  such  that  the  card  can  be  given  a  number  of  adjustments 
to  suit  possible  contingencies. 

Regarding  the  proper  way  tf)  handle  a  skiascope,  the  various 
tutors  in  skiametry  dift'er,  bat  all  are  agreed  that  the  move- 


40 


merits  of  the  mirror  should  be  of  the  slow,  steady,  straight-line 
order,  and  as  free  from  wabbling  and  semi-circular  motions 
as  possible.  When  movements  of  the  mirror  are  attempted 
by  the  hand-tilting  method  it  takes  many  years  of  practice 
before  a  positive  straight-line  motion  in  all  meridians  of  an 
eye  can  be  depended  upon.  But  where  the  movements  are 
made  by  a  body-tilting  method  the  mastery  of  the  mirror  is 
very  rapid,  some  beginners  acquiring  it  almost  perfectly  after 
only  a  few  days  of  practice. 

Fig.  is. 


THE    CROSS     hr.xcket  mirror. 


A  description  of  this  body  method  is  as  follows :  The 
mirror  handle  is  to  be  grasped  near  its  lower  end  when  the 
skiascope  is  held  in  a  vertical  jxisition.  The  elbow  and  arm 
of  the  hand  holding  the  mirror  are  to  be  pressed  tightly 
against  the  side  of  the  body  while  the  upper  and  inner  edge 


MIRRORS 


41 


of  the  metal  disc,  upon  which  the  mirror  is  mounted,  is  to  be 
held  firmly  against  the  side  of  the  examiner's  nose  and  resting 
on  the  eyebrow  in  such  a  manner  that  the  peep-hole  of  the 
mirror  is  exactly  in  front  of  the  operator's  pupil.  With  the 
mirror  handle  held  in  a  rigid  manner,  almost  the  entire  body 
is  made  to  assist  in  giving  the  proper  movements.     The  torso. 

Fig.  16. 


SHOWIXG   MANNER   OF    HOLDING   MIRROR. 

or  trunk,  acting  as  though  it  were  pivoted  at  the  waist,  while 
the  neck  and  heaving  chest  aid  in  the  necessary  motions.  To 
say  that  this  action  involves  a  sort  of  curtsey,  or  bowmg  move- 
ment, might,  perhaps,  add  to  its  description.    Fig.  16  may  also 


42  SCHEMATIC    EVES 

serve  to  give  a  better  idea  of  how  the  skiascope  is  to  be  held. 

Where  an  examiner  is  compelled  to  use  his  left  eye,  instead 
of  his  right  one,  an  excellent  way  is  to  cross  the  right  arm 
over  the  breast  and  in  this  manner  operate  the  mirror  as 
though  the  right  eye  were  in  use.  The  reverse,  of  course, 
applies  to  those  persons  who  are  left-handed,  and  permits  of 
the  graceful  handling  of  the  skiascope  in  place  of  the  awkward 
non-professional  manner  sometimes  acquired  in  other  methods. 

It  is  always  better  for  an  examiner  to  learn  to  work  with 
both  eyes  open  when  locating  the  reflected  light  on  the  face 
of  a  patient.  After  this  location  the  eye  not  in  use  at  the 
peep-hole  should  be  closed  so  as  to  stimulate  concentration,  in 
order  to  sharpen  the  brightness  of  the  fundus  reflex  as  well 
as  to  define  the  shadow's  edge.  The  closing  of  the  unused  eye 
serves  to  obviate  any  discomfort  an  examiner  may  experience, 
caused  by  the  glaring  light  from  the  lamp  in  use.  and  especially 
so  if  the  latter  is  at  close  range. 

The  mirror  light  on  the  patient's  face  ought  not  to  move 
over  an  inch  in  any  one  direction,  and  the  red  pupil  should 
hardly  ever  be  allowed  to  pass  entirely  from  view,  after  once 
being  found.  All  movements  to  be  made  very  slowly,  since 
rapidity  of  motion  often  interferes  with  judgment  as  to  the 
shadow's  direction,  just  as  the  spokes  in  a  wheel  are  found  to 
be  more  diflficult  to  count  when  the  wheel  is  in  motion  than  when 
it  is  stationary. 

SCHEMATIC  EVES.  The  metal  and  pasteboard  model 
eyes  that  are  on  sale  in  all  first-class  optical  supply  houses 
offer  a  most  excellent  means  for  beginners  to  familiarize  them- 
selves with  the  principles  of  both  skiametry  and  ophthal- 
moscopy. These  eyes,  however,  are  freciuently  imperfect  in 
construction,  and  the  printed  scales  attached  to  them  are  often 
unreliable.  As  a  consequence  the  student  is  apt  to  meet  with 
discouraging  results  in  his  initial  cfF(irts  in  using  them. 


SCHEMATIC    EYES  43 

The  cheap  pasteboard  model,  such  as  shown  in  Fig.  17,  is 
usually  found  to  be  just  as  trustworthy  as  the  more  expensive 
ones,  but  all  of  them  require  testing"  before  their  findings  can 
be  implicity  relied  upon. 

Fig.  17. 


A  "queen,"   PASTKl'.OARD,  SCHEMATIC  EYE. 

A  good  way  to  determine  the  accuracy  of  these  models  for 
skiametric  purposes  is  to  have  an  experienced  skiascopisl 
put  them  to  actual  test  by  first  setting  the  scale  at  "o."  and 
then,  if  a  one-diopter  convex  spherical  lens  causes  a  reversal  of 
the  shadow  in  all  meridians  at  exactly  forty  inches  away,  it  is 
quite  safe  to  rely  on  other  findings  made  by  means  of  the  same 
model.  To  prove,  however,  that  the  scales  are  properly  spaced 
it  is  wise  to  first  test  a  few  of  the  numbers  on  each  side  of 
the  "o"  before  depending  upon  them  for  accuracy,  for  in 
optometrical  work  in  general  it  is  so  easy  to  be  wrong  and  so 
difficult  to  be  precisely  right. 

All  kinds  of  ordinary  errors  of  refraction  can  be  artificially 
created  bv  means  of  these  model  eyes  together  with  a  few  trial 
lenses.  For  instance,  if  an  examiner  is  operating  at  a  distance 
of  forty  inches  away,  by  setting  the  model  so  that  it  shows  one 
diopter  of  myopia  and  then  by  adding  a  one-diopter  concave 
cylindrical   lens,   he  can   create   an   error   of   one   diopter   of 


44  REDUCTION    OF   LENSES 

hyperopic  astigmatism,  A  one-diopter  convex  cylinder  can 
be  used  to  produce  myopic  astigmatism  of  equal  amount.  Set- 
ting the  model  to  show  two  diopters  of  myopia  and  then  using 
a  one-diopter  convex  cylinder  lens  will  create  a  compound 
error  of  minus  one-diopter  spherical  combined  with  a  minus 
one-diopter  cylinder,  due  allowance  of  one  diopter  having  been 
made  for  the  working  distance. 

With  the  model  showing  two  diopters  of  hypermetropia,  if 
a  two-diopter  concave  cylindrical  lens  be  added,  the  exact 
compound  quantity  represented  by  this  error  would  be  plus 
two  diopters  spherical  combined  with  plus  two  diopters  cylin- 
drical, and  to  neutralize  it,  skiametrically  at  a  distance  of  forty 
inches  away,  it  would  require  an  added  lens  power  equal  to 
plus  three  diopters  spherical  combined  with  plus  two  diopters 
cylindrical. 

To  illustrate  a  mixed  astigmatic  condition  the  model  can 
be  set  to  show  two  diopters  of  myopia,  and  then  by  adding  a 
minus  two-diopter  cylinder  at  axis  90  an  error  representing 
minus  one-diopter  cylinder  axis  180  combined  with  a  plus  one- 
diopter  cylinder  axis  90  can  be  obtained,  which  would  require 
the  addition  of  this  lens  quantity,  or  its  equivalent,  to  neutralize 
it  by  means  of  the  one-meter  shadow  test.  At  whatever  axis 
the  cylindrical  lens  is  set  the  axis  of  the  artificial  astigmatism 
will  be  in  the  same  meridian. 

REDUCTION  AXD  TRANSPOSITIOX  OF  LENSES. 
In  practical  examination-room  work  with  the  skiascopic  mirror 
it  frequently  happens  that  a  saving  of  time  and  trouble  is 
effected  by  making  a  test  right  over  the  patient's  own  glasses ; 
this  test  resulting,  perhaps,  in  the  discovery  that  a  compound- 
lens  quantity  needs  to  be  either  added  to  or  subtracted  from 
the  lenses  then  in  use.  Upon  neutralizing  these  glasses  it  is 
found  that  they,  too.  are  of  the  so-called  "compound"  type, 
therefore  an  examiner  must  be  possessed  of  knowledge  that 


ri:ductu).\  of  lenses  45 

will  enable  him  to  tell  the  exact  ray-bending  power  of  the 
four  lens  quantities  involved,  and  to  do  it  also  with  ease  and 
without  waste  of  time  or  likelihood  of  making  mistakes. 

In  the  consideration  of  most  problems  there  is  the  unit  or 
lowest  appreciable  quantity  to  be  dealt  with,  so  it  is  with 
lenses.  Speaking  macroscopically,  the  unit  of  all  lenses  is  a 
cylinder.  Therefore,  if  it  is  learned  how  to  combine  these 
cylinders,  after  having  reduced  all  lens  quantities  to  a  cylin- 
drical basis,  the  transposition  of  lenses  will  be  found  to  be  a 
very  easy  task,  no  matter  whether  the  lens  quantities  dealt 
with  number  few  or  many. 

This  principle  is  much  like  the  one  in  the  old  story  related 
of  the  quack  doctor  who  had  two  bottles  of  medicine  with 
which  he  could  cure  all  the  ills  that  flesh  was  heir  to.  His 
plan  was  to  give  doses  out  of  one  bottle  which  turned  every 
ailment  into  fits,  then  the  remedy  in  the  other  bottle  cured  the 
fits,  and  the  patient  got  well. 

To  carry  out  a  similar  procedure  it  must  be  considered  that 
two  cylindrical  lenses  of  like  kind  and  strength  when  crossing 
one  another  at  right  angles  are  equal  to  a  spherical  lens. 
Hence  the  reverse  follows,  that  a  spherical  lens  is  equal  to  two 
cylindrical  lenses  crossing  one  another  at  right  angles,  and 
whose  kind  and  strength  are  the  same. 

In  the  ophthalmic  refractionist's  consideration  of  cylinders 
he  will  never  need  them  at  any  other  than  at  right  angles  to 
one  another,  no  matter  whether  they  are  plus  and  plus  or 
minus  and  minus,  of  the  same  or  unequal  strengths,  or  whether 
they  are  plus  and  minus  or  minus  and  plus,  equal  or  unequal, 
etc.,  etc.  Their  axes  will  always  be  at  right  angles,  and  for  the 
simple  reason  that  if  they  were  crossed  at  any  other  than 
right  angles  their  combined  refraction  would  show  a  sphero- 
cylindrical effect,  which  could  be  duplicated  by  right-angle 
cylinders. 


46  REDUCTION    OF   LENSES 

Now,  in  a  combination  of  cylindrical  lenses  of  unequal 
strength,  but  of  the  same  kind,  it  will  be  seen  that  when  their 
axes  are  at  right  angles  to  each  other  their  combined  refraction 
will  be  equal  to  that  of  a  compound  lens  whose  component  parts 
are  of  a  like  nature :  as  a  plus  one-diopter  cylinder  set  at  right 
angles  to  a  plus  two-diopter  cylinder  is  equal  to  a  plus  one- 
diopter  spherical  combined  with  a  plus  one-diopter  cylindrical. 
The  second  cylinder  in  the  above  case  having  been  robbed  of  a 
quantity  equal  to  the  strength  of  the  first  cylinder  in  order  to 
convert  the  first  one  into  a  spherical  quantity,  the  robbery  is 
noted  and  due  allowance  made  therefor. 

Except  for  purposes  of  analysis,  the  "crossed  cylinder"  is 
never  to  be  generally  employed;  he  who  prescribes  it  other- 
wise only  exhibits  his  ignorance  of  lenses  and  their  uses,  as 
the  function  of  a  lens  is  to  'bend  rays  of  light,  and  it  matters 
little  whether  this  bending  is  done  by  two  cylinders  crossing 
one  another  at  right  angles  or  whether  it  is  accomplished  by 
means  of  a  lens  where  one  surface  has  a  spherical  curvature. 
This  rule  also  applies  to  so-called  "toric"  lenses  where  the 
curves  of  one  surface  of  revolution  are  elipsoidal  in  character, 
made  so  by  having  one  meridian  of  curvature  either  greater 
or  less  than  the  one  at  right  angles  to  it. 

To  crowd  an  examiner's  head  with  arbitrary  rules  is  likely 
to  lead  to  confusion,  so  that  in  the  case  of  transposing  lenses 
it  is  well  to  simplify  the  process  as  much  as  possible.  There- 
fore, imitating  the  method  of  the  quack  doctor  with  his  two 
medicines,  it  will  be  necessary  to  first  reduce  all  lens  quantities 
to  a  cylindrical  basis  and  then  commit  to  memory  two  short 
rules  for  the  transposition  of  those  cylinders.  The  following 
extra  long  combination  may,  perhaps,  serve  to  make  this 
reduction  principle  plainer: 

-f  r.  D.  S.  C  -f  2.  D.  C.  90  C  —  I.  D.  C.  180  C  +  I.  D.  S. 
C  —  2.  D.  C.  90  C  —  2.  D.  S. 


RKDUCTIOX    OF   LENSES 


47 


Here,  it  is  seen,  are  six  lens  quantities  whose  chief  axes  are 
90  and  180  degrees.  After  creating  two  cohimns  all  of  the 
lens  quantities  are  written  in  the  cylindrical  equivalents  whose 
axes  come  under  these  two  headings,  not  forgetting  that  each 
spherical  lens  is  equal  to  two  crossed  cylindrical  ones  whose 
strength  and  kind  are  the  same.  The  following  is  then 
obtained : 

Axis  90 
+  I- 
+  2. 
o. 
-j-  I. 
—  2. 


Axis 

180 

+ 

I. 

0. 

+ 

I. 

0. 

— 

2. 

In  the  axis  90  column  the  totals  are  +  4-  D.  and  —  4.  D., 
which,  of  course,  neutralize  one  another.  In  the  axis  180 
column  the  totals  of  —  3.  D.  less  +  2.  D.  leave  a  remainder  of 
—  I.  D.  axis  180. 

Take  this  cxami)lc  for  instance : 

+  0.50  D.  C.  45  C  +  0.25  D.  S.  Z  +  0.25  D.  C.  135. 
Here  the  two  chief  axes  are  45  and  135  degrees,  and,  pro- 
ceeding as  before,  the  results  are : 

Axis  45 

+  0.50 

+  0.25 

o. 


Axis  135 

0. 

+ 

0.25 

+ 

0.25 

+  0.75  +  0.50 

The  totals  give  one  cylinder  of  +  0.75  axis  45  to  be  crossed 


48  TRANSPOSITION  OF  LENSES 

by  another  cylinder  of  +  0.50  axis   135.  the  symbols  being 
alike. 

Now  another  examjile  in  reduction : 

+  1.25  D.  S.  Z  —  1-75  1^-  ^-  15  C  —  0.75  D.  S.  C  —  0.25 
D.  C.  105. 

Being  reduced,  the  results  obtained  are : 

Axis   15  Axis   105 

+    1.25  +   1-25 

—  1-75  o- 

—  0.75  —  0.75 
o.  —  0.25 


—  1.25  +  0.25 

This  gives  a  total  of  one  cylinder  of  —  1.25  axis  15  being 
crossed  by  another  cylinder  of  +  0.25  axis  105,  the  symbols 
being  unlike. 

In  the  three  examples  shown  all  lens  quantities  have  been 
converted  into  cylindrical  equivalents,  so  that  in  order  to 
master  them  the  two  short  rules  before  mentioned  must  be 
used  for  the  transposition  of  these  cylinders,  and  then  the 
simple  lesson  will  have  been  acquired. 

Rule  Xo.  i. — In  a  combiuation  of  cylindrical  lenses  of  a 
like  character,  such  as  f>lus  and  f^lns.  or  minus  and  minus,  the 
stren,(:th  of  the  weakest  cylinder  should  be  written  as  the 
spherical  quantity  while  the  niFi-ERENXE  between  the  tivo  lenses 
should  be  uritten  as  a  new  cylindrical  quantity,  the  axis  of  the 
stronger  cylinder  i^ofer)iin^^  the  axis  of  the  cylinder  in  conu- 
bination,  thus: 

-\-  0.75  D.  C.  axis  45  C  +  050  D.  C.  axis  135  should  be 
written  as  equal  to  -f-  0.50  D.  S.  C  +  025  D.  C.-axis  45. 

Rule  Xo.  2. — In  a  combination  of  cylindrical  lenses  of  dif- 


TRANSPOSITION  OF  LENSES  49 

fcrcnt  character,  such  as  f^lits  a)ui  tiiinus,  or  )>ilinis  and  ['his, 
the  strength  of  cither  cylinder  can  be  zcritten  as  a  spherical 
quantity  while  the  sum  of  the  tico  cylinders  should  be  written 
as  a  new  cylindrical  quantity,  the  axis  of  the  second  lens  con- 
sidered governing  the  axis  of  the  cylinder  in  combination,  thus: 

—  1.25  D.  C.  axis  15  C  +  0-25  D.  C.  axis  105. 
can  be  written  in  two  ways,  the  better  way  being  to  write  it 
with  the  minus  quantity  first,  so  as  to  obtain  a  periscopic  effect 
in  the  completed  lens,  this  produces : 

—  1.25  D.  S.  C  +  I-50  D.  C.  axis  105. 

Or  an  equal  refractive  quantity  can  be  obtained  by  writing 
it  in  this  way:  -f-  0.25  D.  S.  C  —  i-50  D.  C.  axis  15. 

To  re-transpose  any  of  these  combinations  it  is  only  nec- 
essary to  proceed  by  the  usual  reduction  to  cylindrical  form 
and  then  apply  whichever  one  of  the  two  simple  rules  may  be 
called  for. 

The  world,  metaphorically  sj^eaking.  takes  off  its  hat  to 
the  mathematician.  And  if  the  optometrician  desires  that  def- 
erence be  shown  him,  too,  he  must  acquire  enough  of  mathe- 
matics to  make  him  proficient  in  his  work. 

Procrastination  and  the  plea  of  "no  time  to  take  up  higher 
optics"  will  neither  advance  the  individual  nor  the  calling  to 
which  he  belongs.  Self-education  is  just  as  good  as  any  other 
kind  of  education,  provided  it  accomplishes  its  object ;  there- 
fore let  him  who  desires  to  make  substantial  skiametric 
advancement  remember  that  the  greatest  service  he  can  do 
himself  is  to  thoroughly  master  the  rudiments  of  light  and 
lenses  and  to  acquire  the  ability  to  juggle  with  all  kinds  and 
quantities  of  ray  and  lens  values. 


CHAPTER     ITT. 

Optical  Principles  Involved  in  Skiametry.  —  The 
"Shadow/'  \\'iiat  to  Look  For  and  How  to  See  It,  and 
How  to  Imitate  Its  Action  with  a  Cardboard  Model. 

OPTICAL  PRIXCIPLLIS  OP  SKLIMETKY.  The 
optical  principles  involved  in  shadow-testing  cover  a  wide 
range,  when  all  minor  details  are  followed  out  to  their  extremes. 
This  pursuit,  of  course,  is  not  expected  in  a  book  of  this  kind, 
and  only  those  optical  facts  which  are  essential  to  a  practical 
understanding  of  the  matter  in  hand  will  be  attempted  here. 
But -as  far  as  possible  provable  truths  will  be  made  to  take  the 
place  of  mere  assertions,  and  to  that  end  the  reader's  attention 
will  be  called  to  a  series  of  drawings  representing  phenomena 
whose  action  can  be  easily  shown  by  experiments  made  with 
simple  "smoke  boxes." 

These  boxes  are  not  difficult  of  home  manufacture.  They 
are  to  be  made  of  ordinary  thin  box  board  in  which,  with  a 
narrow  piece  of  electrician's  tape,  one  side  of  the  box  is  to  be 
replaced  with  a  piece  of  plain  glass.  Then  over  a  round  aper- 
ture one  inch  in  diameter  in  front  of  the  box  another  plain 
glass  is  placed,  and  in  one  corner  a  hole  is  made  which  dan 
be  corked  up.  A  hole  is  also  to  be  made  in  the  opposite  end 
of  the  box,  and  this,  too,  is  to  be  fitted  with  a  cork.  Several 
whiflfs  of  tobacco  smoke  can  then  be  blown  into  the  box.  which 
is  thus  made  ready  to  exhil)it  the  action  of  a  beam  of  light  of 
an  intensity  such  as  might  be  had  from  a  partially  hooded 
Welsbach  gas  lamp  placed  a  few  feet  away.     The  rays  from 


OPTICAL   PRINCIPLES 


the  lamp  arc  first  passed  through  a  properly  adjusted  convex 
lens,  in  order  to  render  them  parallel  before  they  enter  the 
box.     In  measurement  the  latter  is  to  be  four  inches  high, 
four  inches  wide  and  ten  inches  long. 
Fig.  1 8. 


A    IlOME-MAUli   ■■-SMOKE      BOX. 

Now,  if  a  beam  of  light  whose  source  is  such  that  its  rays 
are  parallel  is  permitted  to  enter  the  box  shown  in  Fig.  i8,  an 
illuminated  area  will  appear  at  the  back  end  of  the  inside  of 
this  box,  the  diameter  of  which  area  will  be  the  same  as  that  of 
the  one-inch  opening  in  the  front  end  of  the  box. 


Fig 


SHOWING  REFRACTIf)N    BV   LENS  OF   PROPER  FOCUS. 

Remember  that  this  box  is  ten  inches  long,  consequently  in 
box  (Fig.  19),  which  is  the  same  in  every  way,  except  that  a 


52 


OPTICAL   PRINCIPLES 


trial  lens  of  four  diopters  convex  splicrical  has  been  placed  over 
the  front  aperture  and  has  thus  caused  the  refraction  of  the 
beam  as  shown  in  the  drawing. 

Fig.  20. 


SHO\VING  REFRACTION   BY  TOO  STRONG  A  CONVEX   LENS. 

Now%  supi)Ose  that  in  place  of  the  four-diopter  convex 
spherical  lens  a  trial  lens  of  five  diopters  convex  spherical  had 
been  placed  over  the  front  aperture ;  then  the  refraction  would 
be  as  shown  in  Fig.  20.  The  arrow  pointing  to  the  focus,  or 
crossing  point  of  the  rays. 

Fig.  21. 


SHOWING  REFRACTION  RV  TOO  WEAK  A  CONVEX  LENS. 

In  the  box   (Fig.  21)   the  trial  lens  used  is  a  three-diopter 
convex  spherical  and  as  a  consecpience  its  f<Knis  is  outside  of 


OPTICAL    PRIXCIPLi:S  53 

the  box  altogether,  the  Hght  pencil  through  the  smoke  appear- 
ing as  shown  by  the  drawing. 

The  varying  sizes  of  the  light  areas  on  the  back  of  the 
separate  boxes  should  also  be  noted. 

Attention  has  already  been  called  to  the  fact  that  a  cylin- 
drical lens  is,  refractively,  the  one-half  of  a  spherical  one,  so 
that  if  the  beam  of  light  entering  these  boxes  had  been  made 
to  pass  through  a  cylindrical  lens  instead  of  a  spherical  one 
the  beam  would  have  been  refracted  only  in  the  meridian  at 
right  angles  to  the  axis  of  the  cylindrical  lens  used.  The  light 
areas  would  thus  be  lines  of  light  instead  of  circles,  or  spots 
of  light. 

Up  to  this  time  only  the  light  as  it  enters  the  boxes  has 
been  under  consideration.  Attention  will  now  be  turned  to 
its  action  on  emerging  from  them.  Suppose  a  mirror  be 
mserted  inside,  and  on  the  back  end,  of  each  box.  If  this  mir- 
ror was  absolutely  plane  then  many  of  the  light  rays  entering 
the  box  would  return  and  repass  through  the  one-inch  aperture, 
but  as  only  a  few  of  these  rays  have  to  be  dealt  with,  a  selec- 
tion of  an  infinitesimal  spot,  or  point  of  light,  will  be  made  at 
the  edge  of  the  illuminated  area  on  this  imaginary  mirror. 
This  si>ot  should  be  considered  as  a  source  and  a  note  should 
be  made  of  the  rays  reflected  therefrom.  Or,  better  still,  to 
avoid  complicating  matters,  let  a  pin-hole  opening  take  the 
place  of  the  mirror  at  the  back  of  the  box,  and  then  place  a 
lighted  candle  close  to  this  opening  so  that  its  rays  may  enter 
through  this  small  hole  and  be  made  to  behave  in  about  the 
same  manner  as  though  the  candle  were  actually  placed  right 
in  the  very  hole.  Referring  to  the  box  (Fig.  i8)  again,  the 
emerging  rays  under  the  same  conditions  would  leave  the  one- 
inch  ajjcrture  at  the  front  of  the  box  in  a  divergent  manner,  as 
shown  by  Fig.  22. 


54 


OPTICAL   PRINCIPLES 

Fig.  22. 


LIGHT   EMKRGING   UNREFRACTED. 


In  Fig.  19,  owing  to  the  four-diopter  convex  trial  lens  over 
the  front  aperture  having  its  focus  exactly  at  the  source  of  the 
light,  the  rays  would  emerge  parallel,  as  shown  in  Fig.  23. 

Fig.  23. 


RAY.S    EMERGING    PROPERLY    REFRACTED. 


In  Fig.  20  the  emergent  rays,  owing  to  the  fact  that  a 
stronger  lens  is  placed  before  the  one-inch  aperture,  would  con- 
verge to  the  conjugate  focus  of  the  light  source.  Fig.  24 
shows  this  convergence  to  be  equal  to  one  diopter. 


OrTKAI.    PKINC  Il'I.KS 
I'^IG.    2JL. 


EMERGING  RAYS  HAVING  A  CONJUGATE  FOCUS. 

In  the  box  shown  in  Fig.  21  the  lens  used  over  the  front 
aperture  is  too  weak  to  render  the  emergent  rays  parallel,  and 
as  a  consequence  they  will  diverge  as  shown  in  Fig.  25,  the 
divergency  being  equal  to  one  diopter. 

Fig.  25. 


RAYS  EMERGING  DIVERGENT. 

In  Fig  26  the  conditions  are  again  slightly  changed  from 
those  shown  in  Fig.  22.  Here,  for  the  sake  of  emphasizing 
another  optical  fact,  one  end  of  the  box  has  been  pushed  for- 
ward so  as  to  alter  its  depth  from  ten  inches  to  eight  mches. 


56 


OPTICAL   PRINCIPLES 
I^IG.    26. 


RAYS  EMERGING  UN  REFRACTED. 

As  a  consequence,  the  rays  emanating  therefrom  have  a 
divergency  equal  to  five  diopters.  The  placing  of  a  four-diopter 
convex  trial  lens  over  the  front  aperture  of  this  box,  as  was 
done  with  the  box  in  Fig.  23,  would  cause  the  rays  to  still 
emerge  with  a  divergence  e(iual  to  one  diopter,  as  shown  in 
Fig.  27. 

Fig.  2-]. 


^ 

^ 

^^ 

c    — 
0 

=^-=1^==^ 

(^' 

-^r--rz 

W^/ 

^ ibi)/r/idf /VT  -f-i.c. 

RAV.S    EMERGING   I  N.SUFFICIENTLV    REFRACTED. 

If  a  fivf-diopter  convex  trial  lens  had  been  used  over  the 
front  ai)erture,  as  in  Fig.  24,  the  emergent  rays  would  be 
parallel,  as  seen  in  l''ig.  28. 


optical  principles 
Fig.  28. 


57 


RAYS   EMERGING   PROPERLY   REFRACTED. 

But  if  a  three-diopter  convex  lens  were  used,  as  in  Fig.  25, 
then  the  emerging  rays  would  have  a  divergency  of  two 
diopters,  Fig.  29  showing  this  principle. 

Fig.  29. 


RAYS    EMERGING    WITH    DIVERGENCY    INCREASED. 


These  theories  of  the  angle  of  the  emergent  rays  of  light 
can  readily  be  proved  by  using  a  double  smoke  box,  or  one 
placed  in  front  of  the  other,  the  front  one  having  a  glass  end 
as  well  as  a  glass  side.  To  make  this  test  satisfactorily  the 
source  of  light  at  the  back  end  of  the  first  box  must  be  greatly 


58 


OPTICAL   PRINXIPLES 


intensified  in  order  that  the  hght  may  penetrate  the  smoke  and 
enable  its  direction,  or  angles,  to  be  seen. 

It  is  not  a  great  stretch  of  the  imagination  to  consider  a 
series  of  schematic  eyes  whose  various  depths  are  nearly  the 
same  as  the  lengths  of  the  boxes  in  Figs.  i8  to  29.  The 
term  "nearly"  is  used  because,  instead  of  changing  the  lenses, 
as  was  done  with  the  boxes,  it  will  now  be  necessary,  in  order 
to  obtain  similar  results,  to  change  the  depth  of  the  eyes  a  little, 
making  them  correspond  optically  with  the  boxes  shown  in 
Figs.  23.  24  and  2'. 

Fig.  30. 


AN   EMMETROPIC  EYE. 

Comparing  with  the  box  in  Fig.  23,  an  eye  would  have  to 
be  ten  inches  in  diameter  with  its  refractive  media  equal  to 
four  diopters.  This  would  be  an  emmetropic  eve.  as  shown 
by  Fig.  30. 

The  Jk)x  in  Fig.  23  converted  into  an  eye  thirteen  inches 
deep  shows  it  to  be  myopic,  because  the  emergent  ravs  are  con- 
vergent one  diopter  as  in  Fig.  31. 


optical  principles 
Fig.  31. 


59 


a  myopic  eye. 
Fig.  32. 


A    HYPERMETROPIC    EYE. 


6o  OPTICAL   PRINCIPLES 

I-'ig.  2/  becomes  a  hyperopic  eye  on  its  conversion,  as  shown 
by  drawing  Fig.  32.  This,  of  course,  is  pre-supposing  that  the 
accommodation  of  the  eye  is  fully  relaxed. 

In  shadow  testing  it  becomes  necessary  to  have  all  eyes 
virtually  myopic  in  order  to  measure  the  shadows.  Before 
coming  to  the  real  consideration  of  what  is  meant  by  the  tenn 
"shadow."  it  will  perhaps  be  well  to  first  say  a  few  words  about 
this  myopia,  true  and  artificial,  which  has  just  been  mentioned. 
The  myopia  requisite  for  shadow-testing  at  the  usual  distance 
if  forty  inches  is  equal  to  one  diopter,  to  artificially  produce 
which  the  eye  in  Fig.  30  would  require  the  addition  of  a  one- 
iliopter  convex  lens,  as  the  emergent  rays  are  here  parallel.  In 
the  eye  in  Fig.  31  no  trial  lens  would  be  required,  for  these  rays 
already  emerge  convergent  equal  to  one  diopter.  But  if  the 
true  myopia  were  greater  or  less  than  this  quantity,  it  would 
either  have  to  be  raised  or  lowered  to  this  amount,  as  the  case 
might  be. 

In  the  eye  shown  in  Fig.  32  the  rays  diverge  one  diopter,  so 
that  it  would  take  a  one-dio])ter  convex  lens  power  to  alter 
them  to  parallelism,  hence  a  single  lens  of  two  diopters  convex 
would  be  necessary  to  produce  the  required  convergence. 
Thus  it  will  be  seen  that  if  the  convergency  or  divergency  of 
the  emergent  rays  vary  more  or  less  in  angle,  a  stronger  or 
weaker  lens  will  be  needed  to  make  the  rays  first  parallel,  and 
then  after  that  convergent  one  diopter,  in  order  to  lender  them 
of  working  value  at  a  distance  of  forty  inches. 

The  point  has  now  possibly  been  reached  where  something 
may  be  said  understandingly  alxiut  the  shadow.  It  will,  there- 
fore, not  be  out  of  place  to  quote  from  a  little  leaflet  entitled 
"The  Value  of  the  Shadow  Test  in  the  Fitting  of  Glasses," 
which  was  published  by  the  author  some  years  ago. 

"When  rays  of  light  from  a  lamp,  or  other  illuminator,  are 
reflected  into  an  eye  by  means  of  a  mirror  having  a  small  hole 


OPTICAL   PRINCIPLES  6l 

in  its  center,  an  observer,  by  i)lacing  his  eye  at  this  hole,  can 
note  that  the  pupil  of  the  observed  eye  appears  red  instead  of 
black.  If  the  mirror  is  moved  from  side  to  side  a  pale  cloud- 
like shadow  will  pass  or  flit  across  the  red  pupil.  The  behavior 
of  this  shadow  under  certain  conditions  constitutes  a  test 
whereby  optically  defective  eyesight  can  be  accurately  esti- 
mated, and  from  this  estimate  correcting  lenses  can  be  supplied." 
^^S-  33  ""la.v  give  something  of  an  idea  as  to  the  appearance 
of  this  so-called  "shadow." 

Fig.  33. 


ILLUSTRATIXG  A   SO-CALLED      SHADOW. 

To  see  what  causes  it,  and  what  it  really  is,  it  will  be  neces- 
sary to  go  back  to  the  box  in  Fig.  18.  Through  the  smoke  in 
that  box  a  spot  of  light  is  to  be  seen  at  the  back  end  of  the  box. 
The  area  of  this  spot,  as  was  described,  is  that  of  the  one-inch 
aperture  in  the  front  end  of  the  box.  This  spot,  or  area  of 
light,  is  surrounded  by  darkness,  or  shadow.  Now,  if  the 
source  of  light  be  moved  so  that  the  beam  enters  the  box  at 
a  different  angle,  the  spot  on  the  back  end  of  the  box  will,  of 
course,  move  in  the  opposite  direction  to  the  movement  of  the 
light  source.  As  this  spot  moves  so  does  its  edge  or  "shadow" 
move,  too,  and  as  this  edge  under  certain  optical  conditions 
will  assume  a  cloud-like  appearance  which  may  cither  partially 
or  totally  cover  the  red  pupil  of  an  eye.  and  which  seems  to 
behave  in  a  manner  cjuitc  independent  of  the  movement  of  the 
light,  which  is  really  responsible  for  it,  the  early  observers  of 


62  OPTICAL    rUINXIPLES 

these  phenomena  gave  it  the  name  of  "shadow  test,"  whereas 
"light  test"  would  perhaps  be  equally  applicable,  since  without 
a  light  there  can  be  no  edge  to  a  shadow. 

By  reference  to  Fig.  19  it  will  be  seen  that  the  light  area  in 
the  box  is  much  smaller  than  those  in  Figs.  20  and  21,  although 
in  Figs.  20  and  21  the  light  spots  are  apparently  the  same  in 
size  even  though  different  lenses  are  used  to  produce  them. 
This  size  of  the  light  area,  however,  indicates  a  high  or  low 
degree  of  hyperopic  or  myopic  error,  and  the  smaller  the  area 
the  less  will  be  the  error,  besides  the  action  of  the  shadow  will 
be  quicker,  for,  of  course,  a  large  spot  takes  a  longer  time  to 
pass  a  given  point  than  a  small  one  does  provided  the  speed 
is  the  same. 

The  edge  of  the  spots,  or  light  areas,  claim  attention  next, 
and  as  this  edge  is  practically  a  curved  line,  one  side  of  which 
is  dark  and  the  other  side  light,  the  action  of  this  line,  or  edge, 
is  to  be  observed :  the  reader  is  therefore  referred  to  Figs.  23. 
24  and  25,  as  the  emergent  light  will  act  in  a  similar  manner, 
the  edge  of  the  shadow  theoretically  taking/  the  place  of  the 
pin-hole  light  source  in  the  lx)x.  It  is  but  a  step  from  the  eyes 
shown  in  Figs.  30,  31  and  32  to  carry  the  imagination  forward 
to  the  real  thing,  for  living  eyes  are  constructed  on  the  same 
general  optical  principles  as  the  boxes  shown  in  Figs.  18  to  29. 

Now,  to  give  another  illustration  of  the  principles  governing 
the  phenomena  discovered  by  Bowman,  Fig.  34  and  35  may 
serve  to  emphasize  two  salient  points.  In  Fig.  34  it  will  be 
noted  that  the  light  from  the  candle  is  reflected  into  the  eye  and 
converges  towards  an  imaginary  focus  behind  the  retina,  thus 
producing  a  much  larger  area  of  illumination  than  if  the  enter- 
ing rays  of  light  had  been  parallel,  instead  of  divergent,  and 
■were  brought  to  a  focus  exactly  on  the  back  of  the  eye. 

It  mav  also  be  observed  that  the  iris  of  the  eye  serves  to 
<lia|)hragm  the  light  by  permitting  only  such  rays  to  enter  as 


optical  pkin'ciples 
Fig.  34. 


^^ 


SHOWING     HOW     THE    RETIXA     IS     ILLUMIXATED     DV     REFLECTEn 
LIGHT. 

can  pass  through  the  open  pupil,  thereby  causing  the  retinal 
illumination  to  be  round  in  appearance,  the  size  of  this  round 
illumination  being  governed  chiefly  by  the  distance  at  which 
the  candle  and  mirrot  are  placed. 

The  function,  therefore,  of  the  reflected  light  is  merely  to 
illuminate  ?  large  or  small  spot  on  the  retina,  and  this  illumina- 
tion has  practically  nothing  to  do  with  the  estimation  of  any 
error  of  refraction,  as  this  estimation  is  dependent  upon  the 
emerging  rays  of  light  and  not  upon  the  entering  ones. 

Fig.  35  shows  rays  of  light  whose  source  is  at  the  edge  of 
the  illuminated  spot  on  the  mirror-like  retina,  and  it  is  these 
Fig.  35. 


SHOWING    THE    SOURCE    OF    THE    RETURNING    LIGHT    TO    BE    THE 
EDGE   OF   ILLUMINATION. 

rays  that  indicate  by  the  angle  of  their  emergency,  whether 
there  is  any  myopia  or  hypermetropia  present,  for  in  emme- 
tropia  they  emerge  parallel.  An  examiner,  therefore,  with  his 
eye  at  the  peep-hole  of  a  skiascopic  mirror,  has  only  to  deter- 
mine the  angle  of  the  emergent  rays  of  light  by  observing  the 
behavior  of  the  so-called  "shadow"  which  surrounds  the 
illuminated  spot  on  the  retina,  as  this  shadow  manifests  itself 
in  the  pupil  of  the  eye  under  examination. 


64  THE   SHADOW 

THE  SHADO'A'.  To  put  into  words  an  adequate  de- 
scription of  the  shadow  so  that  a  novice  could  recognize  it  at 
once,  even  when  looking  for  it.  is  not  an  easy  task.  The 
reader's  attention  is  therefore  invited  to  Figs.  36  and  37.  which 
illustrate  the  two  halves  of  a  model  by  the  aid  of  which  a  better 
idea  may  perhaps  be  gained  of  the  appearance  of  what  is  called 
the  "shadow"  and  its  action  than  can  be  obtained  by  any 
artificial  means  other  than  with  a  schematic  eye. 

To  make  this  device  the  two  drawings  are  to  be  pasted  on 
cardboard,  so  as  to  render  them  opaque.  The  pupil  of  the  eye 
on  card  (Fig.  37)  and  the  slot  and  hole  on  card  (Fig.  36)  are 
then  to  be  cut  out  with  a  sharp  knife,  after  which  a  piece  of 
pink  translucent  paper  is  to  be  pasted  on  the  back  and  over 
the  pupil  of  card  (Fig.  37).  Then,  with  the  additional  aid 
of  a  lamp  or  candle,  the  paraphernalia  is  complete. 

The  operator  is  to  separate  the  cards  about  a  foot,  in  order 
to  obtain  a  poor  shadow,  and  to  hold  both  in  front  of  a  lighted 
lamp  with  the  card  (Fig.  36)  next  to,  and  at  least  ten  inches 
away  from  it.  so  that  the  illumination  may  not  be  too  bright. 
He  will  then  be  able  to  give  an  illustration  of  a  passing  shadow 
similar  to  that  seen  in  most  eyes  when  viewed  through  the 
peep-hole  of  a  skiascopic  mirror. 

In  comparison  with  these  eyes,  if  the  mirror  is  a  plane  one, 
and  is  operated  forty  inches  away,  the  patient  having  a  one- 
diopter  convex  spherical  lens  in  use.  the  shadow  will  behave  in 
the  following  similar  manner : 

When  the  lamp,  or  light,  is  made  to  move  from  right  to 
left,  both  cards  being  held  stationary  and  in  such  a  wa\  that 
the  edge  of  the  large  hole  in  the  card  (Fig.  36)  will  cast  a 
partial  shadow  over  the  pink  pupil  in  card  (Fig.  37),  thus 
causing  this  shadow  to  move  from  left  to  right,  then  this 
motion  is  said  to  be  "against  the  mirror"  and  it  indicates  a 
myopic  condition  in  an  eye,  which  may  be  either  simple  or 


Fig.  36. 


Pink  paper  for  pasting  on  the  back  of  Fig.  37.  after  the 
pupil  of  the  eye  has  been  cut  out. 


Fig.  37. 


SECOND    HALF  OF    I'ASTFIioARD    MODKL    FOK   l>EMONSTRATI  NG   THE 
shadow's    ACTION. 


THE   SHADOW  69 

compound.  In  simple  myopia,  the  shadow  moves  against  the 
mirror  equally  in  all  meridians.  In  simple  myopic  astigmatism 
there  is  one  meridian  in  which  there  is  no  motion ;  this  is  the 
axis.  All  of  the  other  meridians  show  more  or  less  movement. 
In  compound  myopic  astigmatism  the  movement  of  the  shadow 
is  against  the  mirror  in  all  directions,  but  with  greater  speed 
and  with  less  definition  in  one  meridian  than  in  that  at  right 
angles  to  it. 

In  hypermetropia  the  movements  are  just  the  reverse  of 
this ;  that  is,  when  the  reflected  light  moves  across  the  pupil 
of  the  patient's  eye  the  shadow  will  be  found  to  move  in  the 
same  direction  as  the  movement  of  the  mirror.  To  imitate 
this  with  the  cardboard  models  the  lamp  must  remain  sta- 
tionary while  the  cards  are  in  the  same  position  as  before,  the 
card  (Fig.  36)  is  then  made  to  move  a  few  inches,  thereby 
illustrating  the  movement  called  "With  the  mirror." 

Like  myopia,  hypermetropia  also  can  be  simple  or  com- 
pound. In  simple  hyperopic  astigmatism  there  is  one  merid- 
ian in  which  no  motion  is  observable.  This  is  the  axis.  In 
compound  hyperopic  astigmatism  the  movement  of  the  shadow 
is  with  the  mirror  in  all  directions,  but  unequally  so,  greater 
sped  and  less  definition  being  shown  in  one  meridian  than  in 
that  at  right  angles  to  it. 

The  small  slot  in  the  card  (Fig.  36)  next  to  the  lamp  can 
be  used  to  illustrate  high  degrees  of  astigmatism,  the  long 
diameter  of  the  slot  showing  as  a  band  of  light  on  the  pink 
pupil  and  indicating  the  axis  of  the  error.  Where  this  error 
is  slight  the  band  will  be  so  wide  that  only  one  edge  of  it  will 
apj)ear  in  a  jnipil  at  one  time,  showing  merely  as  a  straight 
edge  and  dififoring  from  a  spherical  error  which  shows  a  slight 
crcscent-likc  curve  at  its  edge. 


CHAPTER    IV. 

Action  or  riii£  Shadow  and  What  It  IxnicATES. — Appear- 
ance OF  THE  Shadow  in  Regular  and  Irregular 
Errors  of  Refraction  and  Its  Action  in  Emmetropia, 
Myopia,  Hvpermetropia  and  Astigmatism. 

THE  SHADOW'S  ACTIOX.  After  a  student  has 
become  thoroughly  famiHar  with  what  is  meant  by  the  term 
"shadow."  as  used  here,  and  is  able  to  observe  the  shadow  in 
livinj]^  eyes  as  well  as  in  schematic  ones,  the  next  question 
which  naturally  arises  is  as  to  what  its  action  indicates  and 
why  the  phenomena  are  not  the  same  in  all  eyes  under  similar 
conditions  of  mirror,  light  and  lenses.  Attention,  therefore, 
is  called  to  Figs.  38  and  39  for  the  purpose  of  making  plain 
the  shadow's  apparently  erratic  movements  in  myopia,  hyper- 
metropia  and  astigmatism,  both  simi)le  and  compound,  includ- 
ing regular  and  irregular  conditions  of  media  and  retina. 

To  illustrate  an  emmetropic  condition,  as  well  as  a  hyper- 
opic  one,  the  somewhat  inaccurate  drawing  (Fig.  38)  is 
intended  to  show  that  the  observed  eye,  P,  is  of  such  a  depth 
that  the  rays  of  light  emerge  from  it  parallel.  If  this  eye 
were  hypermetropic,  and  there  were  no  accommodation,  the 
rays  would  emerge  divergent :  the  effect,  however,  would  be 
the  same. 

Now,  it  will  be  perceived  that  the  dart,  or  arrow,  at  the 
back  of  the  eye,  P,  points  downward.  Taking  a  supposed  ray 
at  the  arrow's  point  it  can  be  followed.  And,  as  it  is  well 
known  that  all  objects  appear  on  the  retina  of  an  eye  just  as 


THE    SHADOW  S    ACTION 


71 


they  do  on  the  sensitized  plate  of  a  camera,  namely,  upside 
down,  or  inverted,  it  will  be  seen  that  the  lowest  ray  which  is 
at  the  point  of  the  arrow,  leaves  the  patient's  eye  as  the  highest, 
or  topmost  one,  and  traveling  to  the  examiner's  eye,  E,  behind 
the  peep-hole  of  the  mirror,  it  enters  this  eye  as  the  upper  ray 
still,  and  then,  undergoing  refraction,  it  is  photographed 
upside  down  and  becomes  the  lower  ray,  finishing  at  the  point 
of  the  arrow  on  the  observer's  retina,  in  the  same  direction  as 

Fig.  ^8. 


SHOWING    WHY    THE    SHADOW    MOVES    WITH    THE    MIRROR. 


when  it  started.  As  a  result  of  this,  its  position  on  the  retina 
of  an  observer's  eye  will  always  be  relatively  the  same  as  its 
position  was  at  its  source,  on  the  retina  of  the  observed  eye,  no 
matter  how  it  is  moved,  provided,  of  course,  that  the  refrac- 
tion of  the  observed  eye  is  not  altered  by  placing  any  lenses 
in  front  of  it,  or,  if  it  is  a  living  eye,  by  increasing  its  accom- 
modation. 

In  myopia  of  a  conjugate  focal  length  less  than  the  work- 
ing distance  at  which  an  examination  is  made,  when  light  is 
reflected  into  an  eye  it  can  be  observed  that  the  shadow  moves 
against  the  movement  of  the  skiascopic  mirror.  Now,  if 
attention  is  directed  to  Fig.  39  (which  is  also  somewhat  inac- 
curate) it  will  be  made  plain  just  why  this  motion  is  opposite 
to  the  movement  of  the  reflected  light  from  the  mirror. 


•J 2  TMK    SHADOW  S    ACTION 

The  mvopic.  or  observed  eye,  I',  as  will  be  seen,  is  of  too 
great  a  depth,  and  consec|uently  the  rays  emerging  from  it 
cross  one  another  before  they  reach  the  examiner's  eye.  E. 
Therefore  the  lower  ray  in  the  observed  eye,  P,  at  the  nrrow's 
point,  emerges  as  the  upper  ray,  as  before,  but  instead  of  con- 
tinuing parallel  it  converges  to  the  conjugate  focus  of  its 
-I'urce.  and  then,  after  crossing  the  other  rays,  it  enters  the 
\e,  E,  as  a  lower  ray,  and  being  refracted  again  it  arrives  at 
lie  retina  of  the  examiner's  eye  as  an  upper  ray.  or  at  the 
jnoint  of  the  arrow,  which,  as  will  be  observed,  is  now  pointing 

Fig.  39. 


snowiNc,  WHY  riii£  shahow  mo\i:s  against  tiif.  mirror. 


in  the  <ip])nsite  direction  to  tliat  at  the  back  f)f  the  observed 
eye.  Consequently,  a  movement  of  a  light  or  shadow  on  the 
retina  of  the  observed  eye.  1'.  would  create  an  opposing  or 
contrary  movement  on  the  retina  of  the  examiner's  eye,  E. 
Thus  the  expression  is  used  that  the  motion  is  "against  the 
mirror."  * 

In  simple  myoi)ia  and  in  simple  hyperopia  these  move- 
ments with  and  against  the  mirror  are  in  all  meridians,  after 
the  working  refraction  is  allowed  for.  In  simj^le  astigmatism 
there  is  motion  in  all  meridians  but  one.  and  that  one  is  the 
axis.  In  all  other  meridians  except  the  one  exactly  at  right 
angles  to  this  axis  the  movement,  instead  of  being  precisely 
with  or  against   the  mirror's  luotion,  becomes  slightlv  erratic 


THE    SHADOW  S    ACTION 


by  showing  an  oblique  action,  or  one  with  a  tendency  to  vary 
a  little  from  the  meridian  in  which  the  mirror  light  is  moved. 
For  instance,  suppose  the  axis  of  the  astigmatism  to  be  J^er- 
tical ;  the  motion  with  or  against  will  then  be  precisely  hori- 
zontal  if  the  mirror  is  moved  exactly  in  the  horizontal  meri- 
dian,  but  if  through  carelessness  or  inexperience  the  light  is 
made  to  travel  obliquely,  say,  fifteen  degrees  above  or  below 
the  horizontal  meridian,  then  the  motion  will  not  be  com- 
pletely neutralized  either  by  means  of  a  spherical  lens  or  of 
a  cylindrical  one  whose  axis  is  vertical.  Therefore,  in  these 
days  of  accuracy,  it  is  important  to  be  as  precise  as  possible, 
and  to  be  careful  to  move  the  mirror  so  as  to  make  the  light 
travel  exactly  in  the  meridian  of  least,  or  of  no,  refraction. 

In  compound  astigmatism  the  movement  of  the  shadow 
will  be  more  defined  in  one  meridian  than  in  another,  and  the 
shadow  will  also  move  with  greater  speed  in  one  meridian 
than  in  another.  The  meridian  of  greatest  definition  or  the 
one  showing  the  darkest  and  heaviest  shadow,  however,  will 
represent  the  meridian  of  greatest  error,  while  the  meridian 
showing  the  shadow  at  greatest  speed  and  least  definition  will 
represent  the  meridian  of  least  error. 
Fig.  40. 


-f  52.  D. 


+  5^-  D- 


+  52.  D. 

ILLUSTRATING    TOTAL   REFRACTION"    IN    EMMETROPIA. 


74 


THE    SHADOW  S    ACTION 


To  illustrate  these  points,  the  student  will  perhaps  be 
helped  by  a  reference  to  the  crosses  in  Figs.  40  to  50,  inclusive. 
It  is  here  to  be  supposed  that  a  perfect,  or  emmetropic,  eye 
has  a  total  convex  refraction  in  all  meridians  of  fifty-two 
diopters,  as  shown  by  the  cross,  Fig.  40. 

An  eye,  therefore,  that  is  one  diopter  hypermetropic  would 
be  illustrated  bv  cross.  Fig.  41. 

l-u;.  41. 


+  51. D. 


+  51.D. 


+  5I-D. 

ILLUSTRATING    A    WEAK    DEGREE    OF    HVPER.METROPIA. 

And  one  of  three  diopters  by  cross.  Fig.  42,  both  being  less 
than  fifty-two  diopters. 

Fig.  42. 


49.  D. 


+  49-  D- 


+  49-  D. 

illustrating   a    MARKED   DEGREE   OF    H  VI'ERMETROPIA. 


THE    SHADOWS    ACTION" 


75 


A  myopic  eye  whose  error  is  one  diopter  could  be  shown 
by  cross,  Fig.  43 

Fig.  43. 


+  53-  D. 


+  53-  D- 


+  53- 1^- 

illustrating  a  wkak  degree  of  myopia. 

Fig.  44. 


+  55-D. 


+  55-D. 


+  55-D. 

ILLUSTRATING    A    MARKED    DEGREE    OF    MYOPIA. 

and  one  of  three  diopters  by  cross,  Fig.  44.  The  refraction  in 
each  being  greater  than  fifty-two  diopters.  In  astigmatism 
cross.  Fig.  45,  shows  one  diopter  of  what  niight  be  termed 
half-hypcrmetropia  in  the  horizontal  meridian.  The  cylin- 
drical lens  required  to  correct  this  error  would  have  to  be  set 
at  an  axis  of  ninety  degrees. 


76 


THE    >ll.\|)()\\   S    ACTION 


Cross,    Fig.    46.    illustrates   one    diopter   of   myopic   astig- 
matism at  axis  one  hundred   and   eighty   degrees.     Each   of 

Fi*^.  45- 


H51.D. 


j.D. 


ILLLSTRATING    II VPERMETROPIC    ASTIGMATISM. 

these  two  optical  conditions  varies  from  the  fifty-two-diopter 
standard  in  one  meridian  only. 

I-IG.  46. 


+ 


D. 


+  53-  D. 

ILLUSTRATING    MVOI'IC    ASTIGMATISM. 

The  compound  hyi)eropic  astigmatism,  illustrated  by  cross, 
Fig.  47,  shows  that  a  lens  of  +  i.  D.  S.  C  +  i.  D-  C.  axis  90 
would  be  required  to  bring  its  refraction  up  to  standard. 


THE    shadow's    action 

Fig.  47- 


17 


;o.D. 


+  51.D. 

ILLUSTRATING    COMPOUND    HYPERMETROPIC 
ASTIGMATISM. 

Cross    Fig.  48,  shows  compound  myopic  astigmatism  re- 

'io„c  nf        T  D   S   -^  —  I.  D.  C.  axis  180  in  order  to 
qmrmg  a  lens  ot  —  i.  i->-  ^-  ^        ^-  -^ 

lower  its  refraction  to  standard. 

Fig.  48. 


+  53-D- 


+  53-  D- 


+  54.  D. 

ILLUSTR.XTING  COMPOUND  MYOPIC  ASTIGMATISM. 

In  mixed  astigmatism  requiring  a  lens  of  +  i-  D.  C.  axis 
00  ^  -  I  D  C.  axis  180,  the  over-refraction  in  one  meridian, 
and^ihe  under- refraction  in  that  at  right  angles  to  it  arc  shown 
by  cross,  Fig.  49. 


the  shadow  s  action 
Fig.  49. 


51- D. 


+  51.D. 


+  53-  D- 

ILLUSTRATING   MIXED  ASTIGMATISM. 

All  these  crosses,  except  that  in  Fig.  40,  are  at  variance 
with  the  fifty-two-diopter  standard  in  one  or  more  meridians. 
Fig.  50  shows  how  the  same  principle  can  he  ilhistrated  when 


-f  55.  D.  -f  49-  D. 

ILLUSTRATING    MIXED   A.STIGMATISM    AT   OBLIQUE   AXES. 

the  axes  of  greatest  and  least  curvature  do  not  lie  in  either  a 
horizontal  or  vertical  position.  As  here  shown,  axis  forty- 
five  indicates  three  diopters  of  myopia,  while  axis  one 
hundred    and    thirty-five    shows    three    dioi)ters   of    hyperopia. 


THE    SHADOW  S    ACTION 


79 


The  case,  therefore,  could  be  classified  as  one  of  mixed  astig- 
matism with  oblique  axes,  the  transposed  formula  calling  for 
a  compound  lens  of  -  3-  D.  S.  3  +  ^>-  D-  C  axis  45-  or  one 
of  +  3.  D.  S.  C  —  6.  D.  C.  axis  135. 

In  cross  Fi?.  40.  if  the  skiametric  conditions  were  made 
perfect  bv  first \ising  a  so-called  "working"  lens,  or  its  ac 
commodative  equivalent,  there  would  be  no  appreciable  move- 
ment of  the  shadow,  no  matter  in  what  meridian  the  mirror 
were  made  to  travel.  In  cross.  Fig.  41.  there  would  be  move- 
ment  with  the  mirror  equally  in  all  meridians.  This  move- 
ment would  be  fairly  rapid,  but  the  shadow  would  not  be  as 
dark  or  as  heavv  as  it  would  be  in  cross,  Fig.  42.  where  the 
error  is  greater.'  In  cross.  Fig.  42.  however,  the  movemen 
of  the  shadow  would  be  more  sluggish,  and  the  color  would 
be  deeper  also. 

In  crosses.  Figs.  43  a.-.l  44-  tl^e  shadow  will  move  aga.nst 
the  movement  of  the  mirror  in  all  n,eri<lians,  otherwise  e 
rules  appheable  to  crosses,  Figs,  4r  an,l  4^.  "^  nearly  the 
same.  In  cross,  Fig,  45-  the  case  is  one  "' . --P'^  ^'simpi 
astigmatism.  In  cross.  Fig,  46.  the  case  .s  one  of  snnple 
mvopic  astigmatism. 

'  In  cross,  Fig.  47-  a  compound  hyperopic  case,  the  moNC- 
ment  is   with  the  mirror   in   all  meridians. 

In  cross  Fig.  49-  the  condition  is  a  mixed  one,  the  oper- 
ato;"::^ng  th^^ertical  meridian  with  a  -vex  ^ a. 
the  horizontal  meridian  with  a  concave  one.  for  in  the  vertical 
[he  motion  is  with  the  mirror,  while  in  the  horizontal  the 
motion  is  against  it,  i,,:^,, 

In  schematic  eyes,  where  everything  .s  large  an,l  br.ght, 
and  wh  re  t      eve  is  perfectly  stationary,  these  findmgs  are 
muci:ea:^r't:a:i;  h„Ln  eyes.    However,  as  the  old  sa.vmg 
coes   -Evervthing  comes  to  him  who  wa.ts     (and  keeps  t> 
f^).    So  with  hmnan  eyes,  they  become  easy  to  "scope    after 


80  THE  shadow's  AIM'KARANCE 

the  student  has  persevered  and  not  given  up  with  an  "I 
can't,"  or  pronounced  the  shadow-test  a  failure  after  a  few 
unintelHgent  efforts  to  master  it. 

APPEARANCE  OP  THE  SHADOW.  The  word 
"regular"  is  defined  by  the  dictionary  as  "acting  according 
to  rule:  following  a  uniform  course,"  etc.  This  definition  is 
very  fitting  when  applied  to,  perhaps,  two-thirds  of  the  cases 
which  present  themselves  for  skiascopic  examination. 

A  typical  "regular"  case  is  one  in  which  the  cornea,  crys- 
talline and  other  media  are  free  from  anything  which  might 
prevent  light  from  passing  from  the  front  to  the  back  of  an 
eye  in  an  unobstructed  or  undeflected  manner,  other  than 
that  called  for  by  the  ordinary  refractive  condition  of  eyes  in 
general.  There  is  also  a  fair-sized  ocular  pupil  and  a  bright- 
colored  retina  in  which  the  light  reflex  shows  a  pronounced 
pink  tint. 

The  word  "irregular"  is  defined  as  "not  regular:  depart- 
ing from,  or  being  out  of,  the  usual  course."  Therefore  a 
thickening  of  the  tissues  of  the  cornea,  resulting  perhaps  from 
inflammatory  processes,  might  give  rise  to  an  irregular  re- 
fractive condition.  This  irregularity  could  also  result  from 
a  disturbance  in  the  transparency  of  the  crystalline,  due  to 
cataractous  conditions,  etc.  One  of  the  most  frequent  causes 
of  unsatisfactory  skiametric  work  lies  in  the  lack  of  brilliancy 
of  the  retina  of  an  eye,  this  dulness  being  attributed  to  an 
excessive  pigmentation  at  the  fundus,  giving  such  a  poor 
reflex  that  the  movement  of  the  shadow  is  most  difiicult  of 
determination  with  any  degree  of  accuracy.  But  as  the  stu- 
dent advances  in  the  judgment  which  comes  only  from  ex- 
perience, these  troublesome  interferences  are  minimized. 

That  the  reader  may  more  fully  understand  the  appearance 
of  the  various  shadowv  reflexes  met  with  in  skiametric  work. 


THE  shadow's  appearaxce 


8 1 


attention  is  called  to  a  few  drawings  which  are  intcn.lcd  lo 
aid  beginners.  Fig.  51  shows  a  shadow  rather  too  pronounced 
to  be  quite  natural,  but  it  serves  to  emphasize  the  crescent-like 

Fig.   qi. 


SHOWING    THE    CRESCEXT-LIKE    SHADOW    IX    SPHERICAL    CASES. 

curve  of  the  advancing  edge  of  the  shadow,  which  indicates 
thafthe  error  is  of  a  simple  myopic,  or  hyperopia  character, 
and  that  it  is  not  astigmatic.  For  if  the  error  were  of  the 
latter  kind  the  shadow  would  show  more  of  a  straight  edge, 
the  principle  of  which  Fig.  52  is  intended  to  illustrate.     This 

Fig.  52. 


SHOWING    THE    STRAIGHT    EDGE    APPEARANCE    OF    THE    SH.ADOW 
IN    ASTIGMATIC    CASES. 


Straight  edge  in  pronounced  degrees  of  astigmatism  takes  on 
a  band-like  form  as  shown  in  Fig.  53-  Injveal^degreethis 
band  is  difficult  to  determine  on  account  of  its  excessive  width, 


82 


THE  SHADOW  S  APPEARANCE 


but  where  the  error  is  two  diopters,  or  more,  then  tlie  axis 
of  the  astiginatism  is  readily  cHscerned  by  noting  the  direc- 

FiG.  53. 


SHOWING  THE  PAND  OPSERVABLE  IN    HIGH   DEGREES  OF 
ASTIGMATISM. 

tion  of  the  band.     Fig.  54  shows  a  marked  error  at  an  axis 
of  45  d'^'grees. 

Fig.  54. 


SHOWING    AN    ASTIGM  \IU      I'.AND    AT    OPLIQUE    AXIS. 


In  operating  at  forty  inches  distance  from  an  eye  of  this 
kind,  when  a  working  refraction  equal  to  one  diopter  is  in 
use,  if  the  mirror  reflected  the  light  afong  the  axis  of  forty- 
five  degrees  there  would  be  no  apjireciable  movement  of  the 
shadow  either  with  or  against  the  mirror's  movement ;  but  if 
the  reflected  light  were  mf)vc(l  along  the  meridian  represent- 
ing axis  one  hundred  and  thirty-five  degrees,  then  the  shadow 
would  show  a  dark,  cloudy  edge  and  would  move,  very  de- 
cidedly in  some  cases,  cither   with  or  against  the  movement 


Tin-:  shadow's  .\rin:.\K.\NCE  83 

of   the  light   which   would   depend,   of   course,   upon   whether 
the  astigmatism  was  of  a  myopic  or  hyperopic  character. 

Fig.  55  is  intended  to  illustrate  the  difference  in  curvature 
between  the  two  edges  of  the  shadow  at  axes  ninety  and  one 
hundred  and  eighty  degrees ;  also  to  show  a  difference  in  its 
intensity,  which  may  be  seen  in  cases  of  compound  hyperopia 
or  myopia.     Of  course,  these  two  shadows  do  not  appear  at 

Fig.  55. 


SHOWING    THE    MERIDIONAL    APPEARANCE    OF    THE    SH.\DOW    IN 
COMPOUND  ASTIGM.VTISM. 

one  and  the  same  time,  but  if  the  mirror  be  rotated  in  the 
vertical  meridian  first  and  the  horizontal  next  it  will  be  seen 
that  the  vertical  movement  shows  a  curved  edge  and  a  lighter 
shadow  than  the  horizontal  movement.  The  horizontal  move- 
ment gives  a  straighter  edge  and  a  deeper  shadow.  A  com- 
pound error  of  +  2.  D.  S.  Z  f  2.  D.  C.  90  wiU  give  a  good 
example  of  this. 

Figs.  51  to  55,  just  referred  to,  are  merely  intended  to 
give  a  crude  idea  of  what  is  meant  by  the  term  "regular" 
conditions,  for  if  an  attempt  were  made  to  give  exact  photo- 
reproductions  of  the  many  eyes  met  with  in  actual  practice 
the  result  would  be  found  most  confusing  and  troublesome 
of  comprehension.  In  Fig.  56  is  shown  what  might  be  called  one 
of  the  "irregular"  kind.  This  depicts  a  double  band-hke  ap- 
pearance of  a  shadowy  reflex  and  is  known  by  the  name  of  the 
"scissors"  movement,  for  instead  of  the  band  passmg  cither 


84 


THF.  SHADOW  S  APPEARANCI£ 


Fig.  56. 


SHOWING    THE    DOUBLE    BAND    IN    THE       SCISSORS       MOVEMENT. 

on  or  off  of  the  pupil  in  accordance  with  the  motion  of  the 
mirror  it  seems  to  close  in  from  both  sides  at  one  time  like 
the  closing  of  a  pair  of  scissors.  The  cause  of  this  may  lie 
either  outside  or  inside  of  an  eye.  If  outside,  it  indicates 
that  the  correcting  lens  used  is  considerably  tilted  in  its  set- 
ting or  else  the  eye  is  viewed  too  far  on  one  side  of  its  anterior- 
posterior  axis;  if  inside,  that  the  crystalline  is  slightly  askew 
in  its  position,  from  some  cause  or  other,  and  that  what  is 
called  a  "cylindrical  equivalent"  is  developed,  one  side  of  this 
so-called  "cylinder"  quantity  being  much  weaker  than  the 
other.    Hence  the  phenomena. 

Another   irregular  condition   is   shown   in    Fig.   57.     This 


Fig 


SHOWING   A    CASE   OF      IRREGULAR      ASTIG.MATISM. 


illustrates  a  simple  astigmatic  case  of  higji  power,  where  the 
cornea  is  so  thickened  by  scar  tissue  that  the  ei\ge  of  the  band 
of  light  and  shadow  shows  an  unevenness  which  is  in  marked 
contrast   to   Figs.    53   and    34.      Of  course,   it    would    not   be 


THE  shadow's  appearance 


85 


expected  that  vision  in  a  case  like  this  could  be  n^ide  to  equa 
twentv-twentieths.  but  the  correction  of  the  error  might  still 
prove   of   considerable   assistance   to   some   poor   unfortunate 
whose  daylight  had  been   somewhat   dulled,   notwithstandmg 
the  presence  of  that  great  benefactor,  the  sun. 

In  Ficr   58  is  shown  an  irregular  condition  due  to  opacities 
in  the  crvstalline.  resulting  in  cataractous  strict.     These  cases 
are  frequently  very  deceptive,  for  where  poor  vision  is  looked 
Fig.  58. 


SHOWING    A    CASE    WHERE    ^CORTICAL    CATARACT"    IS    PRESENT. 

for  good  is  often  found,  and  vice  versa,  so  that  no  matter 
how  many  opacities  appear  it  is  always  wise  to  determine  the 
refraction  skiametrically  if  possible. 
Fig.  59. 


SHOWING  THE  SHADOW  AS  IT  SOMETIMES  APPEARS   IN  "CONICAL 
CORNEA." 


In  Fig  59  an  attempt  has  been  made  to  show  the  appear- 
ance of  a'pupil  where  the  irregularity  of  the  --  -anifes  s 
itself  in  what  is  commonly  called  a  'conical  ^^rn  .  Tl  se 
cases  are  very  difficult  to  do  anything  for.  either  with  or  with 


86  THE  shadow's  appearance 

out  the  aid  of  skiametry.  However,  the  shadow  sometimes 
serves  to  enable  an  examiner  to  obtain  some  clue  which  may 
possibly  be  of  value  in  giving  slight  assistance  by  correcting 
a  part  of  the  refractive  errors  present.  In  such  a  case  the 
shadow  would  be  very  apt  to  show  much  heavier  on  one 
side  of  the  irregular  reflex  than  on  the  other. 


CHAPTER   V. 

Some  Theories  Regarding  the  Dullness  of  the  Fundus 
Reflex  in  Certain  Cases.— Also  Multiple  Meth- 
ods FOR  Practicing  Skiametry,  Including  the 
Toxic  and  Non-Toxic  Manner  of  Employing  the 
Static  Test  by  Both  the  Amplifying  and  Fogging 
Methods. 

SOME  THEORIES  REGARDIXG  REFLEXES.  In 
the  consideration  of  a  subject  as  broad  as  the  term  "oculalr 
skiametry"  would  indicate,  the  temptation  of  an  author  to 
soar  his  kite  in  the  reahn  of  speculation  and  attach  to  its 
tail  a  few  theories  of  his  own  is  very  great  indeed,  even  if 
the  theories  given  should  prove  "bad  pennies"  and  return  to 
embarrass  their  giver. 

In  the  actual  practice  of  skiametry  there  arise  certain 
little  side  details  which  might  be  considered  as  sul>phenomena. 
and  which  exercise  a  more  or  less  important  bearing  upon 
the  system  as  a  whole.  Attention  will  therefore  be  invited 
to  a  few  of  these  points  which  perhaps  savor  more  of  theory 
than  thev  do  of  practice,  but  which,  nevertheless,  seem  to 
answer  natural  questions  likely  to  arise  in  the  mind  of  those 
who  desire  to  know  the  "why"  of  everything  they  are  inter- 
ested in. 

Ocular  pupils  of  a  size  not  exceeding  two  millimetres  m 
diameter,  notwithstanding  an  examiner's  ability  to  magnify 
them,  are  verv  often   troublesome,  and  constitute  a  part  of 


88  DULL  REFLEXES 

the  cases  which  it  is  wise  to  "refract"  by  means  other  than 
by  skiametry.  This  is  especially  true  if,  in  addition  to  the 
smallness  of  the  pupil,  the  fundus  reflex  is  of  the  deeply  pig- 
mented or  dull  kind.  The  reason  for  this  is  due  not  so  much 
to  a  failure  on  the  part  of  the  examiner  to  see  the  pupil  as 
it  is  to  the  fact  that  the  pupillary  aperture  prevents  a  suf- 
ficient volume  of  light  from  entering  the  eye  in  order  to  create 
a  shadow  sharp  enough  in  outline  to  be  readily  measured. 

In  expecting  to  occasionally  find  some  dull  ocular  fundi 
the  examiner  must  not  mistake  high  degrees  of  myopia  or 
hyperopia  for  excessive  pigmentation.  Especially  in  myopia, 
even  in  an  error  of  only  four  or  six  diopters,  will  the  examiner 
be  sometimes  puzzled  to  determine  why  the  reflex  is  so  poor. 

Fig.  6o. 


SHOWING     WHY     THE     Ki:ri.\\l.      I  I.I.I "  M  I  NATION'      IS     LARGE      IN 
MARKED   ERRORS. 


Adding  correcting  glasses,  however,  often  brightens  the  reflex 
in  proportion  as  the  correction  ncars  the  total,  although  at 
the  exact  reversal  point  of  the  shadow  tiie  reflex  may  again 


DULL   REFLEXES 


89 


become  very  unsatisfactory,  as  may  also  the  movements  of 
the  shadow  at  this  time. 

A  slow  motion  must  not  be  mistaken  for  no  motion.  For 
if  Fig.  60  is  looked  at  it  will  be  seen  why  pronounced  degrees 
of  either  myopia  or  hyperopia  make  large  light  areas  on  the 
retina,  thus  requiring  a  longer  time  for  the  shadow  to  come 
into  view  than  if  these  illuminated  areas  were  smaller. 

Fig.  61  also  shows  this  principle  emphasized  in  a  myopic 
eye,  the  returning  rays  from  the  edge  of  the  illumination 
illustrating  the  distance  the  shadow  must  traverse  before 
reaching  the  opposite  side,  this  area  being  much  larger  than 
if  the  eye  was  an  emmetropic  one. 

Fig.  61. 


SHOWING     WHY     THE     SHADOW     MOVES     SLOWLY     IX      M.\RKED 
ERRORS. 


The  fact  that  a  reflex  in  a  given  error  of  myopia  is  much 
duller  than  in  a  corresponding  one  of  hypermctropia  is  due, 
nodoubt,  to  the  greater  distance  tTie  TetuniTng  rays  travel  in 
a  myopic  eye  than  in  an  emmetropic  or  hyperopic  one  before 


90 


DULL   REFLF.XES 


reaching^  the  pupillary  opening,  which,  consequently,  dimin- 
ishes their  number  through  radiation  and  absorption.  Fig. 
62  illustrates  a  theory  of  this.     The  shaded  circles  show  the 

I'lG.  62. 


siiowi.vG  wii'i    rui:  shadow  is  duller  i.\   mvoplv  tii.\x  ix 

A   LIKE  DEGREE  OF    HYPERMETROPL\. 


relative  loss  by  ciuenching,  as  the  divergent  rays  from  a  given 
illumination  on  the  retina  of  a  myopic,  emmetropic,  or  hyper- 
opic  eye  strike  the  inside  of  the  iris,  which  serves  to  diaphragm 
part  of  them  and  prevent  their  further  egress. 

Fig.  63  shows  the  comparative  loss  in  intensity  of  illumina- 
tion in  eyes  whose  myopic  or  hyperopic  error  varies  from 
one  to  sixteen  diopters. 

Brightness  of  reflex,  however,  is  not  all  there  is  to  skiam- 
etry,  for,  owing  to  the  optical  phenomena  called  penumbra," 
it  is  sometimes  i)ossible  to  obtain  a  more  defined  shadow  under 
a  moderate  illumination  than  it  is  under  one  more  intense.  A 
reason  for  this  is  suggested  in  Fig.  64.  But  just  what  part 
the  numerous  pcnumbrre  play  in  interfering  with  the  sharp- 
ness of  demarcation  of  the  sihadow  it  is  difficult  to  ascertain, 


DULL   REFLEXES 


91 


for  the   emerging  light   casts   its   penumbra   at  the   peep-hole 
of  the  examiner's   mirror  just  as   the   entering  light  iloes  at 

Fig.  63. 


->»1 

1. 

^/\ 

\  -^ 

n 

\A 

(    • 
,     1 

a\ 

1     1 

' 

— tH-t- 

w 

/  ^ 

^w 

/ 

showing  relative  size  of  retinal  illumination  in  high 
and  low  degrees  of  myopia.  \ 

Fig.  64. 


SHOWING    THE    OPTICAL     PRINCIPLES     OF     PENUMBRA. 


the  pupil.     And  when  it  is  considered  that  a  round  aperture 
is  being  dealt  with  it  will  be  seen  that  the  conditions  are  even 


92 


DULL   REFLEXES 


more  complex  than  would  ordinarily  manifest  themselves  if 
the  shadow  cast  were  a  central  one  instead  of  being  peripheral. 
Figs.  65  and  66  may  serve  to  make  this  subject  better  under- 
stood, for.  as  has  been  said  before,  a  sharp  shadow  is  a  great 
aid  to  good  work. 

Fig.  6:;. 


SHOWING    OPTICAL    PRIXCIPLES    OF    PEXU.MBRA    DOUni.rCD. 


in  I'ig.  65  the  same  principle  can  be  seen  as  is  shown  in 
Fig.  64,  only  the  conditions  are  doubled,  for  here  there  are 
three  points  on  the  candle  flame  instead  of  two,  the  central 
point  acting  in  a  manner  which  virtually  makes  it  the  same 
as  though  four  points  had  been  selected. 

In  Fig.  66  this  principle  has  been  adapted  to  an  eye  where 
two  points  of  illumination  are  again  shown  whose  source  is 
located  on  the  mirror-like  retina,  for  it  is  only  the  center  of 
the  shadow  that  is  involved,  this  shadow  being  produced  by 
the  iris.  If  either  a  plus  or  minus  lens  is  placed  in  front  of 
this  eye  it  merely  serves  to  refract  all  the  light  in  accordance 
with  the  lens  selected,  the  penumbra  still  remaining  to  add  to 
the  indistinctness  of  the  edge  of  the  shadow. 


MULTIPLE  MLTIIODS  93 

If  this  phenomenon  could  be  overcome  it  would,  secmingl>s 
contribute  much  toward  that  skiametrical  accuracy  for  which 
all  skiascopists  are  striving,  but,  as  has  already  been  shown. 

Fig.  66. 


G    THE    IXTERFEREXCE    OF    PENUMBRA    IN    SHADOW 


SHOWIN 

TESTING, 


if  a  gain  be  made  in  one  direction  a  loss  .s  quite  sure  o  foUow 
„  another,  so  that  examiners  must  be  content  wtl,  fuzzy 
shadows  until  some  master  mind  solves  the  problem  how  to 
tet  rid  of  them  without  interfering  with  the  valuable  pomts 
already  secured  in  dealing  with  other  conditions. 

Operating  at  a  distance,  in  order  to  produce  parallel,  m 
of  the  rays,  seems  at  present  to  be  the  only  means  o    ove 
comL  this  phenomenon,  and  this,  of  course,  bars  out  the  use 
"y  test's  and  methods  which  have  been  found  of  grea 
Lvice  in  uncovering  ciliary  spasm,  latent  errors    e^-  J  " 
it  will  be  seen  that  many  contributing  factors  of  both  success 
anlailure  enter  into  the  problem  of  accurately  es.imatmg 
"e  refractL  of  an  eye  independent  of  a  patient's  intelligence^ 
MULTIPLF  METHODS.    Even  in  the  method  to  wh.cM 


94  MULTIPLE  METHODS 

amincr  attempts  to  induce  relaxation  of  the  muscles  con- 
trolling accommodation  of  an  eye  by  instilling  into  the  cul-de- 
sac  of  this  organ  some  one  of  a  series  of  powerful  toxicants, 
for  the  purpose  of  temporarily  suspending  a  part  of  its  func- 
tional activity,  thus,  for  the  time  being,  partially  transforming 
a  living  eye  into  a  sort  of  schematic  one. 

Xon-medical  examiners,  on  the  other  hand,  attempt  the 
suspension  or  relaxation  of  this  accommodation  by  having 
their  patients  endeavor  to  look  at  some  distant  object  in  order 
to  thus  coax  the  muscles  into  a  condition  of  rest,  and,  in 
further  explanation,  it  may  perhaps  be  truthfully  said  that 
in  many  cases  one  overdoes  the  matter  while  the*  other  under- 
does it.  The  medical  examiner's  overdoing  consists  in  forcing 
the  eye  into  an  abnormal  condition  in  which  co-ordination  be- 
tween accommodation  and  convergence  is  temporarily  de- 
stroyed, this  destruction  depending,  of  course,  upon  the 
strength  of  the  drug  used,  and  the  duration  and  frequency  of 
its  instillation,  as  well  as  upon  the  idiosyncrasies  of  the 
patient.  The  results  obtained  by  measuring  the  refraction 
of  an  eye  while  it  is  in  a  state  which  may  be  called  "local 
intoxication"  would  seem  to  call  for  judgment  of  the  very 
highest  order  to  make  the  theoretical  conform  to  the  practical. 

Regarding  the  non-medical  examiner's  manner  of  using 
the  static  method,  especially  in  those  cases  where  the  nuiscle 
action  is  liable  to  be  particularly  vigorous  or  active,  it  can 
be  likened  to  the  old  story  of  the  blind  leading  the  blind,  for 
the  reason  that  if  the  patient  fails  to  maintain  the  requisite 
*  muscular  relaxation  the  examiner  has  no  means  of  knowing 
what  action  has  really  taken  place,  and  his  findings,  therefore, 
are  likely  to  j)rove  very  unreliable. 

".Spasms  of  accommodation,"  as  they  are  termed,  are 
prr)bal)ly  responsible  for  more  mistakes  being  made  in  the 
nrtii-toxic  manner  of  emj^loying  the  static  method  than  can  be 


MULTIPLE  METHODS  95 

attributed  to  the  carelessness  of  patients  lookino-.  or  trying 
to  look,  at  the  object  or  point  to  which  tlu-ir  attention  has 
been  directed. 

In  dealing  with  these  cases  there  are  two  ways  in  which 
static  skiametry  can  be  used.  One  consists  in  beginning  an 
examination  with  only  that  lens  before  the  patient's  eye  which 
is  necessary  to  produce  the  required  working  refraction  for 
the  operating  distance,  whatever  that  may  be,  and  then,  if 
the  case  is  a  hypermetropic  one,  the  convex  lenses  are  to  be 
gradually  increased  in  strength  until  the  reversal  point  of  the 
shadow  is  obtained. 

If  the  case  is  myopic,  however,  then  an  overcorrection  is 
necessary,  and  the  concave  lenses  used  for  this  purpose  are 
to  be  gradually  decreased  in  strength  until  the  reversal  point 
of  the  shadow  is  found.  This  manner  of  increasing  in  hyper- 
metropia  and  of  decreasing  in  myopia  is  called  the  amplifying 
method. 

Overcorrecting  in  hyperopic  cases  and  undercorrecting  in 
myopic  ones  have  been  termed  the  "fogging"  method.  And 
where  ocular  skiametry  is  performed  in  a  non-toxic  manner 
this  method  of  decreasing  lens  values  in  hypermetropia  and 
of  increasing  them  in  myopia  will  often  prove  of  great  assist- 
ance to  an  examiner,  and  especially  so  if  applied  in  a  binocu- 
lar manner,  for  then  the  co-ordinate  action  of  accommodation 
and  convergence  is  such  as  to  give  the  most  reliable  results. 
This,  of  course,  includes  more  particularly  those  cases  where 
the  age  of  the  patient  is  such  as  to  lead  an  examiner  to  fear 
spasmodic  muscle  action. 

In  cases  of  persons  fifty  years  of  age  or  older,  wherein 
presbyopia  has  a  tendency  to  overcome  spasm  of  accommoda- 
tion, then  static  skiametry  will  frequently  be  found  quite 
trustworthy  by  the  non-toxic  method,  but  where  the  age  of 
the  patient  is  less  than  fifty  years  then  the  toxic  method,  or 


96  -MULTIPLE  METHODS 

one  even  more  reliable  than  either  toxic  or  non-toxic,  must  be 
used  to  determine  true  refractive  conditions. 

In  the  toxic  application  of  static  skiametry  it,  of  course, 
matters  little  whether  the  amplifying  or  fogging  method  is 
used,  for  here  the  accommodation  is  supposed  to  be  in  abey- 
ance and  the  examiner  can  suit  his  own  convenience  in  regard 
to  the  manner  in  which  he  alters  his  lens  quantities.  But  the 
toxic  method  has  disadvantages  along  many  lines  when  it  is 
viewed  from  both  scientific  and  economic  standpoints.  In 
its  scientific  aspect  it  fails  entirely  to  tell  anything  about 
muscle  tension  at  the  reading  point,  leaving  this  to  be  esti- 
mated, or  guessed  at,  by  the  examiner,  while  this  reading 
point,  as  is  well  known,  constitutes  one  of  the  most  important 
ends  for  which  glasses  are  usually  adapted.  Frequently,  too, 
for  distance  purposes  a  medical  examiner  is  led  to  advise 
glasses  from  a  theoretical  instead  of  a  practical  viewpoint. 

All  cycloplegics  are,  of  necessity,  mydriatics  as  well,  and 
the  mydriasis  they  produce  constitutes  a  disturbing  factor  in 
some  cases,  for,  as  has  been  described  by  others,  the  pupillary 
field  becomes  so  enlarged  during  an  examination  that  the  oper- 
ator is  compelled  to  limit  his  observations  to  corneal  zones, 
which  in  turn  add  to  the  skiametric  complications  and  serve 
to  increase  the  difficulties  of  the  method. 

Viewed  from  an  economic  standpoint  the  toxic  method 
tends  toward  the  needless  distress  of  patients,  causes  a  quite 
unnecessary  waste  of  valuable  time  in  waiting  for  the  action 
of  the  cycloplegic,  and  takes  a  foolish  chance,  even  if  only  a 
slight  one,  in  risking  a  possibility  of  blindness  resulting  from 
glaucoma. 

Every  working  distance  at  which  static  skiametry  is  prac- 
ticed, whether  by  toxic  or  non-toxic  means,  really  constitutes 
a  mctho<l  in  it.self,  and  for  the  reason  that  the  nearer  to  a 
patient's  eye  an  examination  is  made  the  more  careful  must 


MULTIPLE  METHODS  97 

the  appearances  of  the  shadow,  as  well  as  the  other  features 
of  the  test  be  studied.  For  instance,  in  an  examination  made 
at  eighty  inches  a  half-diopter  convex  working  lens  quantity 
would  have  to  be  placed  before  the  patient's  eye  in  order  to 
focus  the  parallel  rays  of  light  that  emanated  from  the  retina 
of  an  emmetrope.  Here  the  behavior  of  the  shadow  would 
be  much  quicker,  while  its  color  and  intensity  would  be  more 
pronounced  than  if  the  test  were  made  at  forty  inches  away 
where  a  one-diopter  convex  working  lens  was  used. 

A  test  made  at  forty  inches  might  also  prove  very  satis- 
factory, while  one  made  at  ten  inches,  using  a  four-diopter 
lens,  might  be  anything  but  satisfactory,  even  in  the  same  eye. 
And  thus  it  will  be  seen  that  as  a  student  delves  deeper  into 
the  intricacies  of  skiametry  the  more  complicated  does  the 
system  seem,  and  the  more  manifold  do  its  methods  appear. 

Experience,  however,  does  wonders  in  developing  skill 
and  judgment,  so  that  old  examiners  as  well  as  students  profit 
by  constant  everyday  work,  just  as  old  users  of  the  ophthal- 
moscope improve  by  daily  practice  with  this  valuable  little 
instrument. 


CHAPTER    VI. 

Dynamic  Skiametkv  and  Its  Uses  in  [Mastering  Tonic 
AND  Clonic  Spasms. — The  Importance  of  Visual 
Fixation  in  Obtaining  Bright  Retinal  Reflexes 
and  the  Aid  Derived  from  Using  Independent 
Points,  Together  with  a  Few  Words  About  Ray 
Values. 

DVXAMIC  SKIAMETRY.  The  word  "clynamic"  is 
derived  from  the  Greek  word  dynaniis.  and  signifies  forces 
not  in  equilibrium,  or  motion  as  the  result  of  force,  being  the 
opposite  of  "static." 

Dynamic  skiamctry  then,  as  might  be  readily  inferred,  is 
an  api^lication  of  IJowman's  discovery  made  under  some  kind 
of  force.  This  force  is  the  muscular  one  which  is  familiarly 
known  as  "accommodation." 

While  the  static  method  of  practising  skiamctry  is  one 
where  the  ciliary  muscle  of  an  eye  is  at  rest,  the  dynamic 
method  is  the  exact  reverse  of  this,  and  is  made  while  the 
accommodation  is  exerting  itself  sufficiently  to  readily  accept 
refractive  assistance  up  to  a  point  where  its  relation  with  an- 
other muscular  force  called  "convergence"  is  interfered  with. 

The  principle  involved  is  a  simple  one.  It  is  well  known 
that  a  ])ound  weight  placed  upon  the  shoulder  of  a  sturdy 
man  creates  no  appreciable  burden  or  discomfort.  But  load 
this  same  man  down  to  almost  the  limit  of  his  endurance 
and  then  add   this  i)ound  of  additional  weight  and  its  pres- 


DYNAMIC   SKIAMETRV 


99 


encc  will  be  noticed  at  once.  So  it  is  in  dynamic  skiametry. 
A  call  is  made  for  a  pronounced  increase  in  tension  of  the 
accommoilation  by  having  the  patient  read  a  scries  of  test 
types  placed  on  an  examiner's  brow,  or  attached  to  his  mirror, 
then,  by  varying  this  tension  as  judgment  teaches,  and  by 
being  able  to  easily  supply  the  requisite  artificial  lens  power, 
the  accommodation  is  reduced  to  its  normal  relationship  with 
convergence.  Most  eyes,  no  matter  what  the  age  of  the  patient 
may  be,  will  readily  give  up  the  accommodative  excess  which 
has  been  required  to  maintain  near  vision,  and  this  "excess" 
will  be  composed  of  what  one  has  been  taught  to  call  by  the 
name  of  "spasm,"  or  "latent  hypermetropia." 

The  relationship  between  accommodation  and  convergence, 
if  roughly  estimated,  is  found  to  be  in  about  the  proportion 
of  one  to  three  for  the  two  eyes ;  that  is,  when  the  accommoda- 
tion is  exerted  one  diopter  the  convergence  required,  accord- 
ing to  Hartridge,  is  a  little  over  one  and  one-half  degrees 
for  each  eye.  This,  of  course,  is  subject  to  variation,  for  when 
the  accommodation  equals  ten  diopters  the  convergence  is 
estimated  to  be  slightly  over  eighteen  degrees  for  each  eye. 

To  prove  that  this  relationship  between  these  two  factors 
governing  ocular  adjustment  is  reliable,  and  that  normal  eyes 
cannot  converge  without  accommodating,  nor  accommodate 
without  converging,  the  following  simple  experiment  can  be 
made:  Take  a  person  below  forty  years  of  age  with  known 
emmetropia  and  place  him  so  that  his  eyes  are  exactly  forty 
inches  away  from  the  smallest  test  type  which  he  can  see  io 
read  at  this  distance,  then,  while  he  is  engaged  in  the  act  of 
reading,  add  a  quarter  or  a  half-diopter  convex  lens  quantity 
to  both  eyes  and  the  print  will  be  perceptibly  blurred  or 
dimmed.  In  making  this  experiment  be  absolutely  sure  that 
there  is  no  latent  error  present,  or  the  test  will  not  prove 
reliable. 


100  DYNAMIC  SKIAMETRV 

Xow  this  seems  to  show  that  the  relationsliip  between  these 
two  forces  is  a  quantity  adjusted  finely  enough  to  be  relied 
upon,  for  without  the  control  of  the  convergence  the  accom- 
modation would  have  immediately  relaxed  when  the  lens  was 
added  which  took  away  the  necessity  for  the  amount  of  muscu- 
lar exertion  represented  by  the  fraction  of  one  diopter  which 
the  lens  called  for. 

In  the  consideration  of  the  optical  princi])les  of  static 
skiametry,  as  given  in  previous  chapters,  it  was  required 
that  a  one-diopter  convex  lens  be  employed  when  the  working 
distance  was  forty  inches  away,  so  that  artificial,  or  working, 
myopia  might  be  produced  and  the  returning  rays  from  an 
eye  be  made  to  focus,  or  cross  one  another,  at  the  focal  length 
of  the  lens  used.    This  is  correct  for  this  method,  but  for  the 

Fig.  67. 


"Static  "  Method  . 


SHOWING   F.MERGING  RAYS  BEING   BENT  TO  A   FOCUS  BY   A   TRIAL 
LENS. 

dynamic  method  no  lens  is  required  to  produce  this  artificial, 
or  working,  myopia,  as  the  crystalline  lens  of  the  eye  under 
examination  takes  its  ])lace.  Fig.  67  will  perhaps  make  this 
plainer,  for  in  the  drawing  the  artificial  myopia  under  the 
static  method  is  created  by  means  of  a  trial  lens,  the  eye 
being  emmetropic  and  the  rays  emerging  parallel. 

Under  the  dynamic  method,  Fig.  68,  the  artificial  myopia 
is  created  by  means  of  an  increase  in  the  convexity  of  the 
crystalline  lens,  better  known  by  the  term  "accommodation," 
vision  being  fixed  on  some  nearby  object. 


DYNAMIC  SKIAMETRV  IQI 

This  eye,  the  same  as  in  Fig.  67,  is  emmetropic,  thus 
making  the  two  working-  conditions  practically  equal.  Bv 
the  static  method,  if  the  test  is  to  be  made  at  ten  inches  a 

Fig.  68. 


"  Dyna  MIC  "  Method. 

SHOWING    EMERGING    RAYS    BEING    BENT    TO    A     FOCUS    BY 
ACCOMMODATION. 

four  diopter  convex  lens  power  must  be  used.  By  the  dynamic 
method  this  four-diopter  convex  lens  power  can  be  virtually 
added  to  the  crystalline  lens  of  the  eye  under  examination  by 
advancing  the  brow  card  of  the  examiner  up  to  a  distance  of 
ten  inches  from  the  eyes  of  the  patient,  thereby  causing  his 
accommodation  to  be  exerted  to  this  degree  in  order  that  th^ 
letters  on  the  brow  card  may  be  read  out  aloud. 

To  illustrate  the  value  of  this  method,  and  also  to  show 
its  practical  adaptation,  a  case  will  be  considered  whose  error 
of  refraction  is  two  diopters  of  hypermetropia,  one  diopter 
of  which  is  manifest,  and  one  diopter  latent,  or  in  a  condition 
somewhat  spasmodic.  In  examining  this  eye  at  a  distance  of 
forty  inches,  the  patient  looking  at  some  object  twenty  or 
more  feet  away,  it  is  found  that  the  static  method  shows  one 
diopter  of  hyperopia,  as  it  takes  a  two-diopter  convex  lens 
to  produce  a  reversal  of  the  shadow,  one  diopter  of  which 
representing  the  working  refraction  required.  The  dynamic 
method  being  used  in  this  case,  it  is  discovered  that  when  the 
patient  looks  at  the  brow  card,  forty  inches  away,  a  convex 
lens  of  a  diopter  and  a  half  can  be  added  before  a  reversal 


102  DYNAMIC  SKIAMETRV 

of  the  shadow  is  obtained.  The  examiner  then  advances  so 
as  to  make  the  test  at  a  distance  of  thirteen  inches  and  finds 
that  two  diopters  is  now  the  strength  of  the  reversal  lens 
needed.  Advancing  to  within  ten  inches  of  the  patient's  eyes 
makes  no  change.  Withdrawing  to  forty  inches  again,  it  is 
found  that  very  little  alteration  in  appearance  of  the  shadow 
has  occurred,  unless  the  patient  has  looked  away  in  the  iiican- 
time.  when  the  spasm  will  most  likely  reassert  itself. 

Xow  what  has  taken  place?  The  accommodation  called 
for  by  the  dynamic  method  at  forty  inches  is  one  diopter. 
The  patient  having  two  diopters  of  hypermetropia  had,  there- 
fore, to  make  a  total  accommodative  effort  of  three  diopters 
in  order  to  see  the  letters  on  the  brow  card.  The  examiner 
supplies  refractive"  assistance  until  one  diopter  and  a  half 
of  convex  lens  quantity  has  been  added,  when  the  accommo- 
dation relaxes  to  this  degree,  the  shadow  showing  a  reverse 
movement.  Perhaps  this  case  is  one  where  the  age  of  the 
patient  is  less  than  twenty  years,  general  health  considered 
good,  and  muscle  tension,  or  unconscious  habit  of  exertion 
is  suspected  of  being  vigorous.  A  new  test  is  made  at  a 
distance  of  thirteen  inches,  where  the  total  accommodation 
called  for  is  five  diopters,  of  which  two  diopters  represent 
the  hyperopia  and  three  the  amount  called  for  in  emmetropia 
at  this  distance.  Under  this  burden  the  eye  will  be  found 
to  accept  a  two-diopter  convex  lens  quantity  before  reversal 
will  occur.  Rei)cating  the  test  again  at  ten  inches,  no  more 
relaxation  is  found,  theri-by  ])r(n-in<j^  the  second  finding  to 
be  correct. 

To  analyze  still  furtlier  :  it  may  be  stated  that  at  thirteen 
inches,  where  an  emmetrope  uses  three  dio]iters  of  accommo- 
dation, nine  degrees  of  convergence  are  called  for.  A  patient, 
therefore,  who  is  making  five  diopters  of  accommodative  ef- 


DYNAMIC  SKIA.METRV  IO3 

fort,  ought,  correspondingly,  to  make  fifteen  degrees  of  con- 
vergence, thus  calHng  for  a  distance  of  eight  inches. 

W'hile  both  accommodation  and  convergence  seem  some- 
what elastic  they  nevertheless  appear  to  have  a  tendency 
to  attain  a  standard  co-ordination  when  disturbing  factors  are 
removed. 

Fig.  69. 


SHOW  INC    now    ACCOMMODATION    CAN    BE    MADE    TO    ABSORB    A 
CILIARY    SPASM. 

Fig.  69  is  intended  to  illustrate  the  mechanical  action  of 
a  spasm  of  accommodation  equal  to  a  half-diopter.  The  dotted 
lines  at  A  show  the  position  of  the  lens  when  the  accommoda- 
tion is  at  rest.  The  dotted  lines  at  B,  its  position  when  the 
accommodation  is  equal  to  one  diopter. 

If.  therefore,  the  spasm  or  unconscious  muscle  effort  equals 
a  half-diopter,  and  the  patient  be  required  to  look  at  an  object 
which  calls  for  one  diopter  of  accommodation,  then  the  con- 
scious effort  will,  of  course,  be  equal  to  the  difference,  or  one 


104  DYNAMIC  SKIAMETKV 

half-diopter.  It  can  thus  be  seen  that  a  spasm  may  be  gotten 
rid  of  by  what  might  be  fittingly  termed  absorption. 

In  applying  this  method  to  cases  where  the  spasm  is  likely 
to  be  greater  than  one  diopter,  such  as  in  children,  the  test 
should  be  made  at  a  shorter  distance  than  forty  inches. 
Twenty  inches,  for  instance,  calling  for  two  diopters  of  ac- 
commodation ;  ten  inches,  for  four  diopters,  and  so  on. 

Another  view  for  the  elimination  of  spasm  by  the  dynamic 
method  can  be  taken.  A  man  is  told  that  he  .owes  one  dol- 
lar, which  he  is  desired  to  pay.  He  consults  his  books  and 
finds  that  he  has  already  paid  fifty  cents  of  this  amount.  Then 
all  that  can  be  righteously  collected  of  him  is  the  difference,  or 
the  remaining  fifty  cents.  Now  this  same  man  consults  a 
rcfractionist  professionally.  He  is  required  to  exert  his  ac- 
comniodation  equal  to  one  diopter,  hut  having  a  spasm  of 
a  half-diopter — and  ocular  spasms,  as  it  is  known,  are  really 
unconscious  muscular  eflforts — he  has  therefore  unknowingly 
contributed  a  half-diopter  of  accommodation  which  consti- 
tutes one-half  of  the  amount  requested  of  him.  He  thus  needs 
to  consciously  add  a  half-diopter  more  and  the  request  is 
fully  complied  with.  If  his  obligation,  or  error,  had  been 
higher  the  request  would  have  had  to  be  greater  also,  but 
the  principle  is  the  same,  and  so  when  it  comes  to  actual 
tests  made  by  this  method  it  will  be  fmuid  that  the  theories 
here  expounded  will  be  borne  out  in  actual  practice. 

Spasms  of  accommodation  are  now  generally  classified 
under  two  heads — the  tonic,  or  persistent  muscle  eflfort.  which 
is  often  called  "latent"  hypermetropia.  and  the  clonic,  or 
intermittent  action  of  the  ciliary,  which  is  frcquentlv  met  with 
in  the  objective  estimation  of  both  hypermetropia  and  myopia. 
Both  kinds,  however,  are  factors  in  ocular  skiametry  of  which 
cognizance  must  be  taken,  and  methods  used  whereby  their 
disturbing  influence  can   be  overcome. 


VISUAL    1-IXATIUX  IO5 

It  is  the  rule,  perhaps,  especially  in  persons  over  thirty 
years  of  age,  that  skiametric  findings  made  by  the  non-toxico- 
static  method  are  fairly  reliable,  but  the  drawback  to  the  use 
of  this  method  lies  in  the  inability  of  an  examiner  to  differenti- 
ate his  cases  and  to  tell  in  advance  whether  he  is  dealing  with 
one  wherein  spasm  of  accommodation  is  a  factor  or  not,  until 
after  he  has  used  the  toxico-static  or,  better  still,  the  dynamic 
method.  Also,  to  save  time  and  avoid  errors,  it  is  wise  for 
an  operator  to  use  the  dynamic  method  in  all  cases  under  fifty 
years  of  age ;  and  where  the  patient  is  quite  young  it  is  best 
to  use  this  method  at  as  close  a  point  as  from  ten  to  thirteen 
inches  away. 

VISUAL  FIXATION.  In  the  previous  chapter,  under 
the  heading  of  "Theories,"'  etc.,  the  mention  of  a  subject 
which  has  a  direct  bearing  upon  the  question  of  brightness 
of  the  fundus  reflex,  and  corresponding  definition  of  the 
pupillary  shadow,  was  purposely  omitted.  This  subject  re- 
lates to  the  visual  angle  under  which  examinations  should 
be  made,  and  also  to  the  point  of  fixation  at  which  the  patient's 
accommodation  is  to  be  adjusted  during  these  observations. 
But  as  this  subject  chiefly  concerns  the  dynamic  method,  it 
has  been  left  until  this  chapter. 

As  has  been  remarked  before,  in  effect,  the  point  of  re- 
versal of  a  shadow  is  a  very  difficult  one  to  decide  upon  with 
any  satisfactory  degree  of  exactness,  for  the  reason  that  at 
the  precise  crossing  point  of  the  returning  rays  the  shadow 
is  most  erratic  and  very  difficult  of  determination  as  to  its 
action.  And  as  no  lens  power  can  be  added  to.  or  subtracted 
from,  a  patient's  accommodation  without  interference  with 
this  action,  it  will  readily  be  seen  that  the  examiner's  mirror 
must  be  operated  either  inside  or  outside  of  a  patient's  exact 
point  of  fixation,  if  the  behavior  of  the  shadow  is  to  be 
accentuated. 


io6 


VISUAL   FIXATION 


In  Other  words,  if  a  patient  is  looking  at  a  point  exactly 
fifty-three  inches  away,  and  an  examiner  makes  his  observa- 
tion from  a  point  forty  inches  distant  from  the  patient's  eye, 
the  shadow  will  show  a  hyperopic  movement  equal  to  a  quarter 
of  a  diopter.  On  the  other  hand,  if  the  examiner  views  this 
same  eye  from  this  same  distance  of  forty  inches,  the  patient, 
however,  being-  required  to  look  at  a  card  situated  thirty-two 
inches  away,  the  shadow's  action  will  be  a  myopic  one  of  a 
quarter  of  a  diopter.  Fig.  70  will  perhaps  make  this  plainer, 
for  here  it  will  be  seen  that  the  adjustment  of  the  patient's 
accommodation  was  first  made  for  a  distance  equal  to  three- 
quarters  of  a  diopter,  and  then  for  a  distance  equal  to  a  diopter 
and  a  quarter,  the  observer  being  at  a  distance  of  one  diopter 
in  both  cases,  the  action  of  the  shadow  was,  therefore. 

Fig.  70. 


/.16'D. 


t.D. 


SHOWING    THE    AS.SISTANCE    OFFERED    BY     MULTIPLE    FIXATION 
P0INT.S    IN    DYNAMIC    SKIAMETRV. 


first  with  the  mirror  and  then  against  it,  whereas  if  a  test  had 
been  made  at  one  dioi)ter  the  shadow,  of  course,  would  not 
have  shown  any  motion.  Thus  it  will  be  seen  that  the  method 
is  not  unlike  solving  the  question  of  the  exact  middle  of  a 
piece  of  string  by  simply  doubling  it,  for  this  is  what  it  prac- 
tically amounts  to. 

In  estimating  the  behavior  of  the  shadow  the  advantages 


VISUAL    FIXATION  IO7 

grained  by  an  examiner  in  making-  his  estimate  from  two  or 
three  focal  adjustments  of  his  patient's  accommodation  are, 

Fig.  71. 


THE       CROSS       fixation    STAND. 


of  course,  just  double  or  treble  that  of  making  it  from  a 
single  point.  Besides,  it  frequently  occurs  that  an  examiner 
desires  to  ascertain  whether  any   myopia  present   is  greater 


io8 


VISUAL    FIXATION' 


than  one  diopter  or  not,  before  he  changes  his  lenses.  All 
he  has  to  do,  then,  is  to  advance  his  mirror  nearer  to  his 
patient  while  the  latter  is  still  looking  at  the  fifty-three-inch 
fixation  point.  If  he  finds  that  the  shadow  does  not  reverse 
until  the  mirror  is  advanced  to  within  twenty  inches  of  the 
eye  under  examination,  then  he  knows  that  the  myopia  is  more 
than  one  diopter.  And  if  he  has  to  advance  up  to  ten  inches 
before  obtaining  a  reversal  he  then  knows  that  the  error  is 
nearer  to  three  diopters,  and  so  on. 

Fig.  12. 


SHOWING   PRIMARY   POSITION    FOR   FIXATION    CARD. 


The  means  for  accomplishing  these  results  are  very  simple 
indeed.  Fig.  71  shows  a  fixation  stand  and  the  manner  of 
its  construction,  which,  as  can  be  seen,  is  merely  a  light- 
weight nickel-plated  brass  stand  carrying  a  card  having  let- 
ters of  various  sizes  upon  it.  For  primary  tests  it  is  usually 
placed  about  a  foot  behind  an  examiner's  head,  several  inches 
to  his  right,  and  high  enough  for  its  card  of  test  letters  to 
be  illuminated  by  the  light  source  used.  Fig.  72  showing  the 
relative  position  of  light,  examiner,  patient  and  card. 

In  operation  the  examiner  requests  his  patient  to  first 
look  at  the  red  letters  on  the  fixation  stand,  fifty-three  inches 


VISUAL   FIXATION  IO9 

away.  In  making  his  lens  changes  he  stops  with  the  shadow 
moving  sHghtly  with  the  mirror,  then  he  requests  the  patient 
to  look  at  the  black  letters  on  the  brow  card  attached  to  the 
mirror  frame  or  bracket  of  his  skiascope.  If  he  gets  a  reversal 
of  the  shadow,  he  knows  that  he  is  very  close  to  a  correction 
for  this  distance.  He  then  proceeds  to  finish  the  test  and 
eliminate  the  presence  of  any  ciliary  spasm  by  moving  the 
mirror  and  its  brow  card  nearer  to  the  patient,  and  at  the  same 
time  croii'ding  on  all  the  convex  lens  qnantlty  that  he  possi- 
bly can. 

The  fixation  stand  can  be  used  in  place  of  the  brow  card 
for  the  shorter  focal  tests,  but  it  will  not  be  found  as  con- 
venient. The  greatest  advantage,  however,  to  be  derived 
from  the  employment  of  the  fixation  stand  lies  in  the  angle  at 
which  an  eye  can  be  viewed,  also  in  the  apparent  brilliancy 
and  enlargement  of  the  patient's  pupil,  which  is  due  in  large 
part,  no  doubt,  to  a  corresponding  decrease  in  light  stimula- 
tion. But,  be  that  as  it  may,  the  use  of  the  fixation  stand  in 
most  cases  is  productive  of  much  better  initial  results  than 
accrue  when  its  use  is  dispensed  with.  Especially  is  this  true 
where  the  examiner  is  somewhat  lacking  in  the  experience 
which  comes  only  with  years  of  practice. 

The  double  system  of  cards,  or  fixation  points,  has  an 
added  value  to  an  examiner  from  the  fact  that  the  first  ob- 
servation of  a  patient's  eye  is  sure  to  be  made  under  the  most 
favorable  conditions  requisite  for  a  bright  fundus  reflex.  He 
can  then  govern  himself  accordingly  in  further  examination 
of  the  case. 

If  the  reflex  is  a  bright  one,  the  change  of  vision  from 
stand  to  brow  card  will  not  materially  affect  its  brilliancy. 
On  the  other  hand,  if  the  reflex  is  poor,  then  further  tests 
should  be  continued  by  aid  of  the  stand,  which  being  light 
in  weight,  its  position  is  easily  altered  by  the  examiner. 


no  RAv  vALuii:s 

There  is  another  very  vahiable  feature  connected  with 
the  use  of  independent  fixation  points,  and  that  is  in  the  esti- 
mation of  weak  errors  of  refraction  l)y  instructing  a  patient 
to  look  back  and  forth  from  mirror  card  to  fixation  stand, 
the  distances  of  these  two  from  the  [)aticnt's  eye  being  differ- 
ent. 

RAY  VALUES.  From  the  foregoing  it  will  readily  be 
understood  how  important  it  is  for  an  examiner  to  have  a 
comprehensive  knowledge  of  ray  values  as  well  as  of  lens 
values.  And  by  "ray  values"  is  meant  the  strength  of  the 
lens  that  would  be  required  to  parallel  a  ray  of  light  when 
intercepted  at  any  distance  from  its  source.  Fig.  2  in  the  first 
chajitcr  emphasizes  this  principle. 

In  using  both  static  and  dynamic  skiametry  the  posi- 
tion of  an  examiner  and  the  dioptric  values  of  his 
lenses  in  their  relation  to  his"  patient's  visual  fixation  are 
all  factors  in  the  correct  estimation  of  ocular  errors. 
Thus,  if  a  patient  is  looking  at  an  object  situated 
eighty  inches  distant,  and  a  skiametric  examination  is  made 
at  forty  inches  away,  the  ray  value  at  eighty  inches  may  be 
said  to  equal  a  half-diopter,  while  its  value  at  forty  inches 
is  one  diopter;  one  less  a  half  leaves  a  half,  and  the  shadow 
under  these  conditions  will  behave  as  though  the  error  were 
a  half-diopter  of  hypermetropia. 

If  the  i)atient  were  looking  at  an  object  forty  inches  away, 
and  the  examiner  viewed  the  eye  through  the  skiascope  at  a 
distance  of  eighty  inches,  then  the  reverse  would,  of  course, 
be  the  case,  and  the  error  indicated  would  be  a  half-diopter 
of  myopia. 

All  of  this,  however,  pre-supposcs  that  the  examiner  knows 
just  what  his  patient's  accommodation  is  doing,  and  so,  by 
having  fixation  cards  situated  at  forty,  fifty-three,  eighty,  one 
hundred  and  sixty,  and  three  hundred  and  twenty  inches  away, 


RAY  VALUES  I  I  I 

respectively,  it  would  be  possible  for  an  examiner  working 
at  one  metre  to  observe  the  shadow's  action  in  the  four  quar- 
ters of  a  diopter  of  hypermetropia.  This,  however,  would 
necessitate  the  securing  of  an  especially  intelligent  Jind  obedi- 
ent patient,  together  with  expert  skill  on  the  part  of  an 
examiner. 

Later  on  it  will  be  seen  that  a  much  better  method  can  be 
used,  and  one  where  the  intelligence  of  the  patient  is  not  so 
important  a  factor.  The  same  principle  governing  ray 
values,  however,  can  be  applied  to  shorter  distances.  Thus,  a 
patient  with  a  one-diopter  convex  trial  lens  before  his  eye, 
when  looking  at  a  fixation  card  forty  inches  away  can  be  meas- 
ured for  a  quarter,  a  half,  or  three-quarters  of  a  diopter  of 
hypermetropia.  if  the  examiner  will  carefully  measure  his 
distances  from  his  patient's  eye  so  as  to  hold  his  mirror 
exactly  thirty-two  inches  away  when  he  is  measuring  for 
three-quarters  of  a  diopter,  twenty-six  inches  away  when  he  is 
measuring  for  a  half,  twenty-two  inches  away  when  he  is 
measuring  for  a  quarter,  and  so  on,  the  ray  value  modifying 
the  lens  value  and  the  two  combined  influencing  the  total.  In 
fact,  in  skiametry  the  determination  of  ray  values  can  perhaps 
be  summed  up  by  deducting  the  value  of  what  the  ray  is 
actually  doing  from  what  it  really  ought  to  do  under  the 
circumstances. 

The  possibilities  in  the  way  of  juggling  with  accommo- 
dation, lens  values,  ray  values  and  the  shadow,  are  almost 
limitless,  and  it  is  to  be  expected  that  as  ocular  skiametry  and 
its  multiple  methods  become  better  known  there  will  be  found 
many  more  ways  of  applying  its  principles  which  are,  perhaps, 
not  all  fullv  understood  at  this  time. 


CHAPTER    VII. 

Ocular  Mfscu-:  Action  and  the  Ixfluknxe  of  Habit  Upon 
Accommodation,  Convergence  and  Innervation,  with 
Special  Reference  to  Spasms,  "Latent"  Errors,  and 
the  Use  of  Prisms. 

MUSCLE  ACTION  AND  HABIT.  In  order  to  again 
emphasize  the  strong  points  of  dynamic  skiametry,  it  will  be 
wise,  no  doubt,  to  further  consider  ocular  muscle  action  and 
note  its  relation  to  the  other  factors  which  are  essential  to 
binocular  vision  before  it  can  take  place  with  any  reasonable 
degree  of  ease  or  comfort. 

This  involves  such  questions  as.  what  is  a  muscle?  why 
does  it  act?  etc.  So,  without  making  the  subject  too  lengthy, 
the  dictionary  may  be  briefly  quoted  as  stating  that  "a  muscle 
is  an  organ  composed  of  contractile  fibres  through  the  con- 
traction of  which  bodily  movement  is  effected."  These 
muscles  are  classified  as  being  voluntary,  involuntary  and 
mixed,  the  voluntary  being  subject  to  the  will,  the  involuntary 
acting  independently  of  the  will,  and  the  mixed  conihining  in 
some  degree  the  functions  of  both  the  others. 

All  muscles  are  controlled  or  made  to  contract  by  means 
of  nerves,  and  a  nerve  is  described  as  a  cord-like  structure 
composed  of  delicate  filaments  through  which  sensations,  or 
stimulative  impulses,  are  transmitted  to  and  from  the  brain. 
These  impidses  are  expressions  of  what  is  called  "nervous 
energy"  or  "innervation."  representing  the  active  principle  of 
organic  life,  this  nerve  action  being  dependent  for  its  source 


INFLUENCE   OF    HABIT 


113 


upon  warmth,  food  and  the  orderly  performance  of  the  func- 
tions of  the  body. 

It  will  be  seen,  therefore,  that  this  question  is  much  like 
the  ancient's  description  of  wisdom,  which  was  represented 
as  a  serpent  with  it's  tail  in  it's  mouth,  for  the  reasoning 
resembles  a  circle,  no  beginning  or  ending,  since  the  nervous 
energy  seems  to  reproduce  itself.  In  other  words,  it  takes 
health  to  make  nervous  energy,  and  nervous  energy  to  make 
health. 

Fig.  73  is  intended  to  illustrate  by  means  of  converging 
lines  the  normal  balance,  in  an  emmetropic  eye,  between  the 
muscles  governing  accommodation  and  those  controlling  con- 
vergence. 

Fig.  73. 


EMMET  RO PI  A 


SHOWING    NORMAL    BALANCE    BETWEEN     ACCOMMODATION    AND 
CONVERGENCE    IN    EMMETROPIA. 


Here  it  will  be  seen  that  the  harmony  between  these  two 
factors  necessary  to  binocular  vision  is  in  accord,  and  that  the 
lines  representing  accommodation  and  convergence  meet  at  a 
common  point  on  another  imaginary  line  called  the  "median," 
which  extends  forward  from  midway  between  the  two  eyes, 
the  innervation,  of  course,  being  directed  toward  the  internal 
rectus  and  the  ciliarv  muscle  of  each  eve. 


114 


INFLUENXE   OF    HABIT 


I'ig.  74  is  designed  to  show  a  pair  of  thermometer-like 
tubes,  which  for  convenience  might  be  styled  a  ncnronictcr, 
these  tubes  being  filled  with  an  imaginary  vital  fluid  repre- 
senting the  nervous  energy  required  to  maintain  the  action 
of  accommodation  and  convergence.  The  darts  are  to  call 
attention  to  the  height  of  the  fluid  in  the  two  columns,  and  to 
show  the  relative  amount  of  nervous  expenditure  necessary 
to  maintain  requisite  muscles  effort.  Thus,  in  emmetropia 
the  darts  seem  to  be  equal  and  indicate  that,  whatever  the 
proportionate  relationship  really  is.  it  remains  a  quantity  that 
can  be  considered  as  constant. 

Fig.  74. 


U 


SHOWI.VG      REL.MIVE      [  XXERNATIOX      NECESSARY      TO      BALANCE 
ACCOMMODATION    AND    CONVERGENCE    IN    EMMETR0PI.\. 


In  Fig.  75  may  be  seen  the  disturbed  relation  between 
accommodation  and  convergence  which  would  take  place  in 
hypcrmetropia  if  the  innervation  were  normal,  or  equal,  as 
shown  in  Fig.  74.  Thus  esophoria  and  the  origin  of  con- 
vergent strabismus  can  here  be  surmised. 

In  Fig.  75  the  darts,  A  and  C",  show  that  binocular  vision 
would  be  impossible  un<lcr  these  conditions,  and  so  individuals 
having  eyes  of  this  kind  must  learn  to  either  decrease  con- 
vergence or  increase  accommodation  in  an  independent  man- 
ner.    This  relative  effort  is  shown  by  the  position  of  the  darts 


INFLUENCE   OF    HABIT 


in  the  neurometer,   Fig.  76,  the  nervous   impulse  here   being 
much  greater  for  accommodation. 


Fig.  75. 
HY  PeRMETROPIA 


o.  D.  \ 


SHOWING    LACK    OF    BALANCE    BETWEEN    ACCOMMODATION     AND 
CONVERGENCE    IN    HYPERMETROPIA. 


Fig.  yS. 


SHOWING    THE    UNEQUAL    I X  NKKNATION    REQUIRED    TO    BALANCE 
ACCOMMODATION     AND    CONVERGENCE    IN     HYPERMETROPIA. 

In  Fig.  J-  the  lack  of  harmony  between  darts  A  and  C 
indicate  that  in  myopia  accommodation  must  be  decreased  or 
convergence  increased  before  the  two  can  be  brought  together. 

Fig.  78  shows  a  relative  adjustment  in  innervation  to  which 
nature  undoubtedly  resorts  in  cases  of  this  kind. 


ii6 


INFLUENCE   OF    HABIT 


With  an  understanding  of  the  disturbance  in  innervation 
which  a  hypermetrope  or  myope  has  to  make  in  order  to  con- 
stantly maintain  binocular  vision,  it  is  quite  easy  to  see  how 
readily  an  individual  can  form  habits  of  muscular  action  con- 
trolling accommodation  and  convergence  which  would  prove 

Fig.  -j-j. 
MVOPU 


SHOWING    L.\CK    OF    BALANCE    BETWEEN    ACCOMMODATION     AND 
CONVTERGENCE    IN    MYOPIA. 


Fig.  78. 


u 


SHOWING    THE    UNEQUAL    I N  XKUVATION    REQUIRED    TO    BALANCE 
ACCOMMODATION    AND    CONVERGENCE    IN    MYOPIA. 

most  difficult  of  reformation.  And  this  leads  the  reasoner  to 
a  general  consideration  of  so-called  "spasms,"  "latent  errors," 
"muscular  insufficiencies,"  etc.,  etc. 


SPASMS  117 

It  also  serves  to  emphasize  the  importance  of  keeping 
errors  of  refraction  under  constant  correction,  thereby  enabhng 
individuals  to  form  new  ocular  muscle  habits  which  will  be  in 
accord  with  standard  visual  requirements. 

Then,  too,  it  will  show  the  wisdom  of  postponing  the 
adaptation  of  prisms  until  after  old  refractive  habits  have  been 
broken  uj)  by  changing  the  conditions  responsible  for  them. 

SPASMS,  ETC.  Spasms  are  defined  as:  "Involuntary 
convulsive  contraction  of  muscles,"  and  "convulsion,"  in  turn, 
is  described  as  "irregular  and  violent  commotion."  Now,  this 
latter  definition  might  perhaps  be  modified  a  little  and  thereby 
convey  a  somewhat  better  understanding  of  the  term  "spasm," 
as  it  is  used  here. 

Owing  to  the  contractile  quality  of  muscles,  they  can  only 
do  one  thing,  and  that  is  to  pull,  not  push.  Therefore,  when 
the  term  "violent  commotion"  is  used,  it  is  immediately  asso- 
ciated with  the  idea  of  a  rapid  and  intense  muscular  activity, 
which  resembles  the  seizures  of  epilepsy,  and  wrong  impres- 
sions are  apt  to  be  formed. 

As  before  stated,  spasms  can  be  of  two  kinds ;  the  steady 
and  tile  vacillating.  So,  where  the  word  "tonic"  is  used  in 
connection  with  them,  they  are  to  be  associated  with  the  idea 
of  an  unconscious,  involuntary,  regular,  steady  and  persistent 
muscle  tension. 

Where  the  word  "clonic"  is  used,  however,  it  should  be 
understood  to  mean  unconscious,  involuntary,  irregular,  con- 
tracting and  relaxing  muscle  action,  which,  when  taken  in 
connection  with  the  so-called  "accommodation"  of  an  eye.  may 
be  either  rapid  or  slow  in  its  operation. 

The  term  "tonic  spasm  of  accommodation."  as  understood 
to-day,  bears  a  close  resemblance  to  what  many  readers  of 
Bonders  are  led  to  infer  from  his  use  of  the  term  "latent"  in 
connection  with  hypermetropia.   for  the  word  "tonus."   from 


Il8  SPASMS 

which  the  word  "tonic"  is  derived,  signifies  an  involuntary 
condition  which  might  be  Ukened  to  increased  vigor  or  "tone" 
in  a  muscle  whereby  it  may,  perhaps,  insist  on  doing  more 
work  than  is  really  intended  for  it.  This,  of  course,  is  alike 
applicable  to  myopic  and  emmetropic  eyes,  though  not  occur- 
ring with  the  same  frequency  as  in  hypermetropic  ones. 

The  cause  of  these  spasms  is  also  a  varying  one.  In  hyper- 
metropia  the  constant  muscle  tension  required  in  order  to 
maintain  vision  is  no  doubt  a  pronounced  factor.  In  myopia 
and  in  emmetropia  an  uncontrollable  supply  of  nervous  energy 
actuating  the  muscles  is  probably  one  reason  for  their  invol- 
untary contraction.  Occupation,  too,  which  leads  to  the  for- 
mation of  muscle  habits,  is  undoubtedly  another  cause,  and  so 
quite  a  number  of  reasons  might  be  suggested.  The  work  of 
the  refractionist,  however,  deals  only  with  the  determination 
of  their  presence  and  the  estimation  of  their  strength  or  influ- 
ence on  the  refraction  of  an  eye. 

In  a  previous  chapter  reference  has  been  made  to  the 
physician's  use  of  a  local  toxicant,  technically  termed  a  "cyclo- 
plegic,"  which  he  employs  to  paralyze  the  muscle  action  of 
an  eye.  but  the  drawbacks  attendant  upon  the  use  of*  this 
means  are  so  many,  both  from  a  practical  and  from  a  theoret- 
ical standpoint,  that  some  method  had  to  be  devised  in  order 
to  eliminate  the  guess  work  which  must  necessarily  enter  into 
all  cases  where  the  exact  refraction  of  an  eye  at  its  reading  or 
working  point  is  not  known.  And  by  "exact  refraction"  is 
meant  the  muscle  tonus,  or  muscle  innervation,  which  consti- 
tutes a  personal  factor  in  each  patient. 

By  subjective  methods,  if  the  patient  is  of  sufficient  intelli- 
gence and  shows  the  proper  amount  of  interest  in  the  case,  a 
cross-examination  in  connection  with  other  tests  will  enable 
an  examiner  to  form  a  fairly  satisfactory  idea  of  the  range  of 
that  patient's  accommodation,  and   from  this  known  range  he 


J 


SPASMS  119 

can  make  a  close  estimate  as  to  the  strength  of  the  glasses 
required.  By  the  method  known  as  "dynamic"  skiametry, 
however,  a  more  certain  estimate  can  frequently  be  arrived  at, 
for  by  this  examination  the  burden  of  judgment  is  shifted 
from  the  shoulders  of  the  patient  to  those  of  the  examiner 
who,  as  his  experience  increases,  should  be  a  better  judge  of 
his  patient's  needs  than  the  patient  himself. 

In  the  dynamic  method  attention  has  been  called  to  the 
fact  that  where  the  muscles  controlling  accommodation  were 
heavily  taxed  it  was  easy  to  see  how  readily  they  might  be 
induced  to  surrender  any  excess  of  energy  which  they  had 
hitherto  concealed,  no  matter  whether  this  concealment  were 
total,  in  the  form  of  a  tonic  muscle  effort,  or  intermittent  in 
the  form  of  a  clonic  one. 

Attention  will  be  invited  in  the  next  chapter  to  the  advan- 
tages to  be  derived  from  the  use  of  mobile,  rather  than  of 
fixed,  lens  values,  and  this  may  serve  to  make  this  point 
plainer,  for  spasms  of  accommodation,  especially  of  the 
"clonic"  variety,  requires  a  lens  action  which  -vill  in  a  measure 
imitate  the  increase  and  decrease  in  convexity  of  the  crystalline 
lens  of  an  eye  itself.  To  deal  successfully  with  spasms,  an 
examiner  must,  therefore,  use  some  means  which  will  enable 
him  to  make  his  lens  changes  in  a  gradual,  rapid  and  accurate 
manner,  and  at  the  same  time  operate  at  any  desired  distance 
from  his  patient. 

Then,  too.  if  habits  are  to  be  broken  up.  the  patient  must 
be  required  to  make  some  refractive  adjustment  that  is 
unusual,  and  differing  from  the  manner  in  which  his  eyes 
have  been  generally  employed. 

Many  persons  with  uncorrected  hypermetropia  use  their 
eyes  as  little  as  possible  for  reading  or  near  work.  Their 
muscle  adjustment  for  distance,  however,  is  almost  a  constant 
quantity   during   their   waking   hours.     If   such    a   person    is 


:2o 


young,  and  the  ocular  muscle  action  is  vigorous,  a  non-toxic 
test  made  at  twenty  feet  is  very  likely  to  uncover  only  a  por- 
tion of  the  error,  for  the  reason  that  the  accommodation 
muscles  have  been  so  in  the  habit  of  exerting  themselves  at 
this  distance  that  when  lenses  are  supplied,  which  remove 
the  necessity  for  this  exertion,  these  muscles  seem  to  refuse 
much  of  the  assistance  offered  them. 

On  the  other  hand,  if  this  case  is  examined  skiascopically 
at  a  distance  nearer  than  that  of  the  usual  reading  point,  it 
will  be  observed  that  the  eye  will  now  accept  lenses  very  much 
stronger,  for  here  habit  is  not  such  a  factor,  and  under  the 
burden  of  increased  muscle  effort  the  accommodation  will 
readily  give  up  the  excess  tension  which  it  was  so  accustomed 
to  exert  for  distant  uses.  Thus,  habit  being  temporarily 
broken,  the  muscles  governing  accommodation  and  converg- 
ence are  more  likely  to  assume  standard  relations,  and  espe- 
cially so  if  a  lens  system  is  used  which  permits  of  binocular 
mobile  action. 

In  myopia,  particularly  in  those  cases  which  have  remained 
uncorrected  for  a  number  of  years,  the  accommodation  is 
likely  to  prove  disappointing  in  its  action,  by  behaving  either 
spasmodically  or  quite  the  reverse.  For  this  reason  greater 
care  must  be  exercised,  when  skiascoping  these  cases,  to  employ 
corroborative  methods.  A  myopic  case  can  seemingly  be 
readily  changed  into  a  hypermetropic  one  by  over-correction 
of  the  error,  but  muscle  habits  formed  in  ordinary  occupations 
cannot  be  changed  in  a  minute,  and  this  fact  must  ever  control 
the  judgment  of  an  examiner  and  cause  him  to  adopt  such 
methods  and  measures  as  will  give  him  the  fullest  information 
regarding  the  requirements  of  each  and  every  case  that 
presents  itself. 

Of  the  two  forces,  accommodation  and  convergence,  the 
former  is  i)robal)ly  the  controlling  one.     So-called  "muscular 


insufficiencies"  are,  therefore,  doubtless  the  result  of  inco- 
ordination between  the  ciliary  and  the  recti  groups.  If  the 
theories  as  illustrated  by  Figs.  73  to  78,  inclusive,  are  correct 
ones,  then  the  fallacy  of  adapting  prisms  for  the  purpose  of 
permanently  assisting  the  muscles  controlling  convergence  is 
made  plain  for  many  cases,  for  where  the  error  of  refraction 
is  not  fully  corrected  it  is  logical  to  expect  that  the  muscles 
controlling  convergence  will  not  behave  in  accordance  with 
standards  which  are  applicable  only  in  emmetropia. 

Where  the  recti  muscles  are  mal-attached  to  the  eye-ball, 
or  where  there  is  a  permanent  deficiency  in  the  amount  of 
their  innervation,  then  prisms  would  seem  to  be  indicated,  but 
experience  teaches  that  these  cases  are  rare.  It  is  wise,  then, 
for  an  examiner  to  correct  his  patient's  refraction  first  and 
wait  for  the  muscles  governing  accommodation  and  con- 
vergence to  adapt  themselves  to  the  new  order  of  things.  If 
they  fail  to  do  this,  and  the  examiner  is  sure  that  the  error 
of  refraction  is  fully  corrected,  then,  and  only  then,  is  he 
warranted  in  the  use  of  prisms,  unless  for  the  accomplishment 
of  some  temporary  result. 

While  adduction,  abduction  and  sursumduction  appear  to 
oflfer  important  standards,  with  which  to  aid  an  examiner's 
judgment,  yet  they  are  of  little  avail  until  after  emmetropia 
has  been  established  and  maintained  long  enough  for  accom- 
modation, convergence  and  innervation  to  have  ordinarily 
acquired  a  habit  indicative  of  normal  muscle  balance. 

To  emphasize  these  points,  it  can  be  plainly  seen  that  if 
the  relative  proportion  of  three  degrees  of  convergence  to  one 
diopter  of  accommodation  is  taken  as  a  standard  of  estimation, 
then  eyes  having  one  diopter  of  hypermetropia  in  reading  at  a 
distance  of  sixteen  inches  away,  which  calls  for  two  and  a  half 
diopters  more,  must  make  an  accommodative  effort  equal  to 
three  and  one-half  diopters  in  all. 


122 


This  ciliary  action  would  call  for  a  harmonious  converg- 
ence equal  to  about  ten  degrees,  and  ten  degrees,  in  turn, 
would  call  for  a  reading  distance  of  eleven  inches  away,  thus 
showing  that  while  the  eyes  were  accommodating  for  sixteen 
inches  they  would  really  have  to  converge  to  a  point  five  inches 
nearer  in  order  to  use  a  standard  or  proportionate  amount  of 
nervous  energy ;  and  as  this  would  be  quite  impossible  without 
disturbing  binocular  vision,  the  only  other  course  left  is  for 
the  innervation  of  the  convergent  muscles  to  be  correspond- 
ingly decreased. 

As  emmetropic  eyes  cannot  alter  their  co-ordinate  muscle 
action  without  previous  training,  it  would  seem  to  be  reason- 
able to  assume  that  ametropic  eyes  ought  also  to  require  an 
equal  time  and  training  in  order  to  enable  them  to  convert 
their  latent  errors,  or  those  due  to  habits  of  suppression,  into 
manifest  ones,  or  those  within  the  individual's  own  control. 

To  what  extent  habit  interferes  with  the  accuracy  of  static 
skiametry  must  be  left  to  the  imagination  of  those  examiners 
who  have  not  yet  had  opportunities  of  personal  observation. 
And  while  dynamic  skiametry  offers  a  marvelous  improvement 
in  estimating  the  true  refractive  errors  of  young  persons,  it,  too, 
is  frequently  interfered  with  by  this  same  disturbing  factor  of 
habit,  or  "individual  equation,"  as  it  is  sometimes  called. 

It  must  not  be  overlooked,  however,  that,  even  after  latent 
errors  have  been  determined,  and  spasms  unlocked,  it  is  not 
always  wise  to  endeavor  to  force  eye  muscles  to  accept 
standard  relationship  in  too  short  a  period  of  time,  for  while 
it  is  true  that  usually  the  best  results  are  obtained  after 
cmmctropia  and  orthophoria  have  been  established,  yet  it  is 
equally  true  that  in  some  cases  it  is  better  to  ignore  a  small 
spasm  and  permits  the  muscles  to  work,  for  a  while,  in  excess 
of  normal  requirements. 

This,  therefore,  indicates  that  optometrical  findings  are  one 


SPASMS  123 

thing  and  the  judgment  used  in  adapting  glasses  is  quite 
another.  For  with  the  experienced  examiner  it  often  happens 
that  in  youthful  cases  he  may  find  an  error  of  perhaps  four 
diopters  of  hypermetropia,  by  the  dynamic  method,  and  yet 
correct  only  two  di6pters  of  this  in  the  first  glasses  given, 
taking  a  year  or  so  of  time  in  order  to  educate  the  eye  muscles 
up  to  a  normal  standard  where  they  can  bear  a  full  correction 
of  the  refractive  errors  present. 

The  making  of  the  original  or  primary  examination  in  a 
thorough  and  reliable  manner  will  thus  be  seen  to  greatly  assist 
an  examiner  in  prognosticating  his  cases,  as  well  as  in  giving 
advice  the  correctness  of  which  will  be  borne  out  in  due  course 
of  time. 

There  is  an  old  adage  which  says  that  "it  takes  a  long 
time  to  teach  old  dogs  new  tricks,"  and  the  reason  for  this  lies, 
perhaps,  in  the  fact  that  before  new  habits,  or  "tricks,"  can  be 
formed  the  old  ones  must  be  broken  up,  and  this  in  sonte 
cases  may  be  more  of  a  task  than  even  the  formation  of  the 
new.  Spasmodic  muscle  action,  while  unimportant  in  some 
cases,  is  quite  the  contrary  in  others,  so  that  the  wise  examiner 
will  take  no  chances,  but  proceed  to  thoroughly  master  all  of 
his  cases.  Any  unsatisfactory  results  that  may  occur  can 
then  be  attributed  more  to  faulty  judgment  than  to  a  lack  of 
knowledge  of  the  optical  and  correlated  intrinsic  and  extrinsic 
muscle  action  involved. 


CHAPTER    VIII. 

Corroborative  Measurements  in  Optometry  and  the 
Inter-Dependence  of  Objective  and  Subjective 
Methods. — The  Value  of  Mechanical  Devices  and 

THE     SuPERUiRITV     OF     THE      MoBILE     OvER     THE     UnIT 

Action  of  Lenses. 

CORROBORATIVE  MEASUREMENTS.  There  was 
a  time  when  to  possess  an  optometer  of  simple  make,  or  a 
modest  trial  case,  seemed  to  be  all  an  examiner  needed  in  order 
to  cope  with  the  requirements  of  his  cases.  That  was  in  the 
days  when  a  refractionist's  conscience  was  supposed  to  trouble 
him  if  he  gave  a  correction  of  less  than  three  diopters  of 
convex  spherical  lens  quantity  or  recognized  less  than  one 
diopter  of  astigmatism. 

Thanks  to  the  efforts  of  the  progressive — which  includes 
patients  as  well  as  examiners — that  time  has  gone  by,  and 
accuracy  and  attention  to  detail  are  the  order  of  the  day  in 
optometrical  procedure. 

To  do  high-class  work  at  the  present  time  (and  he  who 
does  not  do  it  is  pretty  sure  to  be  left  in  the  race)  an  optomet- 
rical specialist  must  be  thoroughly  familiar  with  the  various 
methods  and  devices  which  have  received  the  stamp  of 
approval  from  those  of  recognized  ability  in  this  field. 

At  first  thought  it  would  seem  as  though  the  trial  case 
ought  to  be  given  primary  attention  on  account  of  its  age,  but, 
logically,  it  comes  last,  because  it  offers  the  nearest  approach 


CORROBORATIVE    MEASUK1-,.\1 1..N  I  >  I  25 

to  an  actual  pair  of  spectacles  or  eye  glasses,  and  because,  also, 
it  gives  the  only  means  of  determining  binocular  vision  with 
any  degree  of  satisfaction. 

To  ocular  skiametry,  however,  belongs  the  first  place  in 
the  refractive  scale,  not  perhaps  from  its  priority  of  discovery, 
but  rather  from  a  utilitarian  standi)oint.  It  is  the  great  path- 
finder and  points  the  way  for  other  work.  In  its  most 
approved  application  it  discloses  minute  opacities  of  the 
cornea  and  crystalline  lens,  thereby  giving  information  at  once 
which  the  ophthalmoscope  could  not  locate  except,  possibly, 
after  a  long  time-consuming  hunt. 

It  tells  of  the  presence  of  astigmatism,  its  character  and 
approximate  axis,  and  also  whether  it  is  complicated  with  any 
error  requiring  the  correction  of  spherical  lenses. 

It  shows  refractive  conditions  independent  of  the  patient's 
age,  language  or  answers,  and  serves  to  check  carelessness  in 
all  persons. 

In  children  it  is  of  invaluable  service,  and  in  those  whose 
hearing  is  faulty  it  saves  much  shouting  and  misunderstanding. 
Its  use,  therefore,  comes  at  the  beginning  of  an  examina- 
tion, and  if  astigmatism  of  over  one  diopter  is  disclosed  it  is 
a  source  of  satisfaction  at  least,  though  perhaps  not  absolutely 
necessary,  to  use  a  keratometer  and  endeavor  to  objectively 
locate  the  exact  axis  of  the  error.  Then,  following  these 
methods,  it  is  well  to  ascertain,  subjectively,  of  course,  whether 
vision  is  in  harmony  w'ith  refraction;  if  it  be  found  otherwise, 
then  the  ophthalmoscope  should  be  employed  to  ascertain,  if 
possible,  why.  and  thus  enable  the  optometrician  to  know 
whether  the  case  is  one  calling  for  glasses,  for  medical  treat- 
ment, or  for  both. 

If  the  vision  and  refraction  agree  in  a  monocular  manner, 
but  not  in  a  binocular  one,  then  phorometric  devices  are  to  be 
called  into  requisition.     And.  therefore,  it  will  be  seen  that  in 


126  CORROnORATUE    MEASUREMENTS 

the  order  of  their  use  skiametry  is  first,  the  keratometer  sec- 
ond, the  trial-case  third,  and  then,  if  needed,  the  ophthal- 
moscope fourth,  and  the  phorometer  fifth. 

Three  of  these  methods  represent  objective  means,  and 
two  represent  subjective  ones.  It  will  thus  be  seen  that  prac- 
tically all  the  five  methods  are  interdependent,  the  only  one 
which  might  really  be  dispensed  with  being  the  cornea 
measure,  and  this  is  not  advisable. 

In  all-round  optometrical  work  the  placing  of  sole  depend- 
ence upon  one  method,  one  device,  or  one  system,  for  success, 
is  about  as  foolish  as  it  would  be  to  place  like  dependence 
upon  one  method,  one  device,  or  one  system  in  the  practice  of 
any  other  semi-mechanical  calling,  where  the  conditions  are 
likely  to  vary  in  diflferent  cases. 

Then,  too,  the  use  of  examination-room  ajiparatus  whose 
only  value  is  to  mystify  patients,  and  make  them  believe  they 
are  undergoing  a  thorough  scientific  examination,  is  a  means 
hardly  calculated  to  maintain  that  lasting  public  confidence 
which  usually  contributes  to  a  long  and  increasing  practice. 
\or  is  it  wise  to  idle  away  a  patient's  time  in  needless  visual 
tests  merely  for  the  purpose  of  trying  to  create  favorable 
impressions  regarrling  professional  ability,  for  there  is  now 
enough  that  is  of  real  value  in  optometrical  work  to  gain,  with 
intelligent  use.  the  confidence  of  educated  as  well  as  unedu- 
cated patrons. 

To  attain  the  very  highest  order  of  jiractical  scientific 
results  should  be  the  well  defined  aim  of  those  who  devote 
either  all  or  part  of  their  time  and  ability  to  the  mastery  of 
physiological  optics.  And  as  a  means  to  this  end  the  practice 
of  systematically  corroborating  all  ocular  measurements  will 
be  found  to  act  as  a  preventive  to  the  making  of  those  mis- 
takes which,  when  discovered  by  some  other  examiner,  are  so 
difificult  of  explanation. 


VALUE   OF    INSTRUMENTS  1 2/ 

VALUE  OF  IXSTRUMENTS.  The  value  of  insiru- 
ments  in  optometry  can  hardly  be  over-estimated,  if  accuracy, 
encouragement  and  speed  are  to  be  considered.  For,  while 
it  is  true  that  all  optomctriciins  should  be  so  trained  as  to  be 
able  to  do  their  work  without  apparatus  other  than  crude 
affairs,  it  is  also  true  that  they  should  be  educated  in  the  expert 
handlings  of  such  devices  as  tend  to  make  their  work  more 
efficient. 

All  ojitometrical  instruments  are  at  best  only  tools  which 
depend  for  their  usefulness  upon  the  intelligence  of  those  who 
handle  them.  The  confession,  therefore,  of  inability  to  use 
an  instrument  is  tantamount  to  a  confession  of  incompetency. 
Some  tools,  or  instruments,  have  greater  scientific  and  eco- 
nomic value,  of  course,  than  others  have,  and  it  frequently 
happens  that  the  instrument  or  device  whose  manipulation  is 
easiest  to  acquire  is  not  always  the  best  one  to  use.  The  inex- 
perienced should  be  influenced  by  the  experienced  in  the  selec- 
tion of  their  examination-room  armament,  provided  this  expe- 
rience is  adequate  and  its  possessor  does  not  belong  to  that 
class  of  examiners  who  get  into  ruts  and  are  incapable  of 
extricating  themselves,  no  matter  what  the  value  of  the  opto- 
mctrical  inducement  offered  to  them. 

In  the  selection  of  instruments  there  is  one  point  which 
usually  commends  itself  to  those  who  have  had  opportunities 
of  using  various  kinds,  and  that  is  the  superiority  of  mobile 
action  over  the  unit  action  of  lenses.  This  so-called  "unit" 
in  the  ophthalmic  lenses  generally  used  to-day  is  termed  "one 
dTo])tcr."  but  in  reality  the  unit  comes  nearer  to  being  a  quarter 
of  a  diopter,  as  lenses  are  employed,  these  "quarters''  acting  a^ 
a  kind  of  step  whereby  the  accommodation  of  an  eye  is  com- 
pelled to  jump  from  one  adjustment  to  another. 

To  overcome  this  jumping  principle  in  the  measuring  of 
convergence  the  "Risley"  mobile  prism   was  invented.     This 


128 


VALUE   UF   INSTRUMENTS 


prism  arrangement  serves  to  change  these  abrupt  degree- 
jumps  into  a  sort  of  slithng  motion,  thereby  permitting  a 
gradual  increase  or  decrease  in  the  Hght  deviation,  and  resem- 
bhng  the  action  of  a  wedge  while  being  made  to  lift  a  weight, 
or  split  a  log. 

vVith  both  cylindrical  and  spherical  lenses  this  sliding  i)rin- 
ciple  can  be  imitated.  Thus  two  convex  lenses  of  four 
diopters  each  when  placed  close  together  on  their  optical  axes 
give  a  combined  refraction  of  eight  diopters,  with  a  focal 
power  equal  to  five  inches.  Sei)arate  these  two  lenses  two 
inches  and  their  combined  action  will  rc])resent  nine  dioi)ters 
of  refraction,  having  a  focal  distance  of  four  and  a  half 
inches.     The  increasing  or  decreasing  of  the  distance  between 

Fig.  79. 


SHOWING    THE    OPTICAL    PRINCIPLES    OF    MOBILE    LENS    ACTION 
PRODUCED  BY  CHANGING  THE  RELATI\E  POSITION  OF  LENSES. 


any  series  of  lenses  on  their  optical  axes  serves  to  produce  a 
mobile  effect  similar  to  the  action  of  the  crystalline  lens  in  a 
living  eye.  thus  enabling  accommodation  to  be  given  the  same 
character  of  assistance  as  that  accorded  to  convergence  by  a 
mobile  prism.  Fig.  jq  will  perhaps  make  this  point  plainer. 
That  this  mobile  action  is  a  valuable  factor  in  dealing  with 


VALUE   OF    INSTRUMENTS  129 

stubborn  cases  of  spasm,  is  not  difficult  to  perceive,  any  more 
than  it  is  difficult  to  note  the  fact  that  to  raise  or  lower  a  heavy 
safe  slidii^s:  on  a  plane  is  easier  than  it  would  be  by  using^  a 
flight  of  steps.  The  evenness  of  the  sliding  motion  serves  to 
induce  an  accuracy  of  action  that  can  hardly  be  imitated  by  a 
motion  which  may,  perhaps,  be  described  as  "jumps"  or 
"leaps." 

An  eye  might  be  considered  as  having  an  error  equal  to  one 
and  three-quarters  diopters,  whereas,  the  true  error  might 
really  be  one  and  five-eighths  diopters  or  one  and  seven- 
eighths,  the  one  and  three-quarters  merely  indicating  an 
approximate  correction.  In  many  cases  this  would  undoubt- 
edly be  near  enough,  but  in  others  a  knowledge  of  the  exact 
refraction  is  often  of  importance.  Where,  therefore,  a  mobile 
lens  system  is  employed  there  is  a  greater  tendency  toward 
precision  than  where  lens  units  are  relied  upon. 

Then,  too.  the  letting  down,  or  relaxation,  of  accommoda- 
tion will  be  found  much  easier  with  a  mobile  lens  system  than 
it  will  be  with  a  unit  system,  for  in  reality  the  mobile  prac- 
tically amounts  to  the  measuring  of  a  living  eye  by  means  of 
an  artificial  one  possessing  equal  refractive  qualities  of  adjust- 
ment. The  invention  of  a  single  mobile  lens  capable  of  adapta- 
tion to  the  needs  of  mankind  would  undoubtedly  prove  a  great 
boon,  but  until  such  an  invention  appears  reliance  will  have  to 
be  placed  on  a  series  of  lenses  arranged  for  the  accomplish- 
ment of  similar  purposes. 


CHAPTER    IX. 
Tin-:   UsK  of   Instkl'.mi:xts    i.\    Ski.\.mi:tkic   Work.    Thf.ir 

EVOLUTIOX     FROM     SiXGLE     LeXSE.S    AXD     THE     RELATIVE 

Merits  of  So.me  Now  Employed,  Ixcludixc.  a  Descrip- 
Tiox  OF  THE  Author's  Own  Mechaxical  Coxtribu- 
Tioxs  i.\  This  Line. 

USE  OF  IXSTRUMEXTS.  After  the  shadow  test 
had  been  improved  and  brought  to  a  state  approaching  prac- 
tical usefuhiess  by  such  men  as  Cuignet.  Parent  and  others,  it 
was  found  that  its  appHcation  entailed  such  frequent  changing 
of  trial  lenses  as  to  be  very  much  like  hard  labor,  and  for  this 
reason  it  was  therefore  neglected  by  many. 

Some  sage  has  said  that  "Genius  is  lazy.""  but  this  is  sup- 
I^oscd  to  apply  only  to  the  "other  fellow."  Anyhow,  it  was 
found  that  if  this  grand  test  was  to  be  advanced  and  made 
I)opular  some  means  would  have  to  be  devised  in  order  to 
encourage  its  use.  The  first  effort  of  importance  along  the 
line  of  labor-saving  devices  for  skiametric  work  appears  to 
have  been  the  lens  rack  to  which  the  name  of  Wiirdemann  is 
frc'(|uently  attached,  although  its  invention  is  claimed  bv  a 
number  of  others,  l^'ig.  80  depicts  the  principle  upon  which 
this  instrument  is  constructed. 

As  will  be  seen,  it  consists  of  a  series  of  concave  and  con- 
vex spherical  lenses  mounted  in  such  a  manner  as  to  permit 
the  patient  to  hold  them  before  his  own  eye  and  move  them 
from  one  to  another,  at  the  request  of  the  examiner.     Inas- 


WUltDKMANN  S  LKXS  RMK  I3I 

much  as  it  proves  a  saving  of  time  and  labor  this  device  is  a 
decided  improvement  over  the  use  of  single  lenses  placed  in 
a  trial  frame.  Among  the  drawbacks  to  its  use,  however,  will 
be  found  the  stupidity  antl  carelessness  of  patients,  who  fre- 
quently permit  the  lenses  to  rest  at  angles  which  interfere  with 
their  correct  refraction,  thus  necessitating  a  constant  readjust- 

FiG.  80. 


Tllli    SKIASCOI'IC    LEXS    RACK    OF    WURDEM ANN'. 

ment  of  the  rack  by  the  examiner.  Then,  too.  owing  to  this 
same  stupidity  and  carelessness,  a  patient  will  often  allow  the 
lenses  to  come  in  contact  with  the  skin  on  his  forehead,  eyelids 
or  cheeks,  and  thereby  soil  the  lenses  so  they  become  ujifit  for 
use  until  after  they  have  been  cleaned. 

Quite  a  number  of  modifications  of  the  W'iirdemann  prin- 
ciple has  been  produced,  but  the  drawbacks  just  mentioned 
seem  common  to  them  all. 

The  Grain  and  the  Standart  discs  of  lenses  have  been  some- 
what popular:  the  first  named  being  on  a  stand  for  use  on  a 
table,  while  the  last  named  is  held  by  a  wall  bracket.  Fig.  8i 
shows  the  large  disc  of  lenses  used  in  both  of  these  devices. 

The  lenses  of  the  disc  can  be  either  spherical  or  cylindrical 
as  the  examiner  chooses.  The  disc  can  also  be  revolved  by 
the  examiner  or  by  the  patient  at  the  former's  request.  These 
discs,  or  batteries  of  lenses,  are  something  of  an  improvement 
on  the  hand  rack  of  Wiirdemann,  inasmuch  as  they  hold  the 
lenses  in  such  a  position  as  to  permit  of  no  material  twisting 


132 


GRAIN   AND   STANDART   DISC 


or  disturbance  to  their  principal  axes.  The  reUance  on  the 
patient  to  turn  the  discs  and  the  necessity  for  troublesome 
adjustments  as  to  height,  together  with  the  annoyances  inci- 

FiG.  8i. 


THi;  DISC   FORM   OF   HOIJJING  LENSES  USED   BY   CRAIN,   STANDART 
AND  OTHERS. 


dent  to  keeping  the  head  of  the  patient  so  situated  that  tiie 
eyes  will  always  be  in  proper  position,  constitute  some  of  the 
reasons  why  these  devices  have  not  been  in  greater  demand. 

The  instrument  of  Jennings  is  based  on  the  principle  of  a 
Morton  ophthalmoscope  in  its  arrangement  for  bringing  lenses 
before  a  patient's  eye.  It  is  also  a  stand  or  table  instrument, 
its  constructive  outline  being  shown  in  Fig.  82. 

In  [)oint  of  improvement  it  seems  to  have  one  very  decided 
advantage  over  the  Grain  and  Standart  types,  as  shown  here, 
and  that  is  in  the  rod  controlling  the  lenses  so  that  the  exam- 
iner can  make  changes  without  depending  upon  the  intelli- 
gence of  his  patient. 

Following  this  instrument  come  those  of  Hamilton  and 
Fav.     These  cmbodv  the   disc   form   of  mounting  the   lenses. 


JENNINGS'    DEVICE  133 

with  a  rotl  for  their  control.     The  improvement  tliey  sliow  lies 
principally  in  having  the  discs  douhlc  in  nnmber,  so  that  more 


Fig.  82. 


THE   COXSTRUCTIVE   PRlNCirLE  OF   THE   "jEXXIXGs" 
INSTRUMENT. 

lens  changes  are  possible.  Fig.  83  shows  the  main  points  in 
the  Hamilton  instrument  which  is  now  known  as  the 
"Meyrowitz  Refractometer." 

Fig.  84  illustrates  the  "Fay"  instrument. 

The  discs  of  this  device  arc  arranged  for  binocular  use. 
while  its  lenses  are  mounted  at  the  end  of  spoke-like  rods 
giving  the  discs  an  api)oarance  similar  to  that  of  a  carriage 

wheel. 

In  its  binocular  feature  the  "Fay"  instrument  mipresses 
one  as  being  superior  to  the  other  devices,  for  they  are.  with 
the  exception  of  the  Hamilton  form,  all  monocular  m  the 
arrangement  of  their  lenses. 


134  REFRACTOMETKRS   OF    MEVROWITZ    AND   FAY 


Fig.  83. 


THE  CONSTRUCTIVE  PRINCIPLE  OF  THE  "MEYROWITZ 

instrument. 
Fig.  84. 


THE    CONSTRUCTIVE    PRI.VCIPLE    OF    THE    "fAV"    INSTRLTMENT. 


author's  instrument  of  1892 


135 


Following  the  instrument  of  Fay,  is  the  instrument  of  Dr. 
Chalmers  Prentice,  now  known  as  the  "Geneva,"  and  patented 
about  1898.  This  instrument  marks  something  of  a  departure 
from  the  forms  that  have  heretofore  been  considered,  for  in 
addition  to  carrying  a  battery  of  lenses  it  also  carries  its  own 
light  and  mirror.  As  an  illustration  of  how  it  is  possible  for 
many  minds  to  be  possessed  of  similar  ideas,  attention  is  called 
to  a  device  which  was  used  in  1892,  and  which  is  still  in  the 
author's  possession.  Fig.  85  gives  an  illustration  of  its  con- 
struction. As  can  be  seen  it  is  merely  a  long  box  containing 
a  stationary  lamp,  a  tilting  mirror  and  two  discs  of  lenses  con- 
trolled by  a  double  rod. 

Fig.  8s. 


THE   CON.STKUCTINE    |-R1\(  II'I.H    oK    TllK    INSTRUMENT    DESIGNED 
.\NU   USED   I!V    TIIK   AUTHOR    IN    1892. 


The  principle  of  the  "Geneva"  instrument  is  shown  in 
Fig.  86,  and  will  be  seen  to  consist  of  a  stationary  lamp,  a  tilt- 
ing mirror,  and  two  discs  of  lenses  controlled  by  a  double  rod. 


136 


THE  GEXE\A   KEIIXOSCOI'E 


The  chief  difference  between  the  two  devices  lies  in  the   fact 
that  one  is  in  a  box,  while  the  other  employs  a  tube. 

Fig.  86. 


THE   CONSTRUCTIVE    PRINCIPLE   OF   THE      GENEVA      INSTRUMENT 
PATENTED  ABOUT  1 898. 

With  the  exception  of  the  author's  new  instrument,  which, 
as  will  presently  be  seen,  is  a  radical  departure  in  almost  every 
way  from  those  just  referred  to,  this  completes  the  list, 
althoujjh  there  are  still  quite  a  number  of  other  instruments 
which  embrace  the  principles  here  shown. 

Jackson  in  his  valuable  book  on  '''Skiascopy,  in  speaking 
of  the  use  of  disc  lenses  (page  109)  says :  "But  even  in  this 
case,  the  fact  that  there  is  a  complete  break  between  the 
appearances  represented  by  one  lens,  and  the  appearances 
present  by  the  use  of  the  lens  next  stronger  or  weaker,  makes 
the  information  obtained  less  valuable  and  satisfactory  than 
that  derived  from  the  movement  of  the  surgeon's  eye  from, 
one  position  to  another,  which  allows  him  to  watch  the  dif- 
ferent appearances  of  light  and  shade  as  they  pass  gradually 
into  each  other."  This  "movement  of  the  surgeon's  eve"  to 
which  he  refers  has  since  been  practically  duplicated  by  mak- 

♦The  Edwards  &   Docker  Co.,  publishers,   Pliila.,   1895. 


THE  ri:tixo-skiameter  137 

ing  the  lens  action  mobile  instead  of  fixed,  thus  securing 
superior  results  with   less  effort. 

From  the  instrument  of  Grain  to  that  of  the  "Geneva"  it 
may  be  observed  that  the  proper  adjustment  of  the  patient's 
head  by  means  of  chin  rests,  etc.,  is  an  important  item,  and, 
with  the  exception  of  the  "Fay"  instrument,  all  require  sep- 
arate adjustment  for  each  eye.  Besides,  not  one  of  them  is 
arranged  for  magnifying  the  patient's  pupil,  no  matter  how 
small  it  may  be  nor  how  great  the  difficulty  of  accurately 
noting  the  action  of  its  shadow. 

The  Jennings,  Hamilton,  Fay  and  Geneva  types  are  all 
designed  to  work  at  one  fixed  distance,  no  latitude  being 
allowed  unless  the  instruments  are  operated  in  an  awkward 
manner,  and  this  of  course  limits  their  use  to  the  static  method 
only,  barring  out  the  dynamic,  which,  of  the  two,  is  really  the 
more  valuable.  Therefore,  all  single  test  instruments  are  of 
little  service  in  thorough  skiametric  work. 

Wiirdemann's  device  is,  like  the  use  of  trial  lenses,  most 
excellent  in  theory,  but  in  practice  its  shortcomings,  seem  to 
be  many.  In  the  practical  workings  of  all  disc  devices  their 
use  usually  proves  of  considerable  assistance  to  an  examiner, 
especially  when  the  disc  can  be  controlled  by  rods,  but  all  disc 
instruments,  it  is  to  be  feared,  will  eventually  be  relegated  to 
the  company  of  the  many  other  optometrical  devices  which 
have  been  weighed  in  the  balance  of  actual  experience  and 
found  wanting.  However,  it  is  for  others  to  judge  of  this 
after  they  have  informed  themselves  regarding  the  advantages 
gained  by  the  use  of  the  next  instrument  to  which  attention 
is  here  called. 

THE  "CROSS"  RETIXO-SKIAMETER.  This  little 
device,  among  its  other  features,  was  designed  to  accomplish 
a  purpose  similar  to  that  of  the  various  disc  contrivances  in 
overcoming  the  necessity  for  having  examiners  change  posi- 


138  THE    RETIXO-SKIAMETER 

tion  every  time  a  lens  needed  changing.  Like  Wiirdemann's 
device,  too,  it  was  designed  for  a  liand  instrument,  so  as  to 
make  the  adjustments  rapid  and  easy,  but  unHke  Wiirde- 
mann's rack,  it  was  arranged  to  be  so  placed  that  its  position 
should  be  as  secure  as  that  of  a  trial  frame. 

Enlargement  of  ocular  pupils  was  sought  for  and  achieved, 
together  with  mobile  lens  action.  Variety  in  methods  of  oper- 
ating was  not  so  much  a  first  consideration  as  it  was  an  after- 
thought.    The  construction  of  the  instrument,  however,  needed 


Fig.  87. 


d    o  o    o 


3H. 


f:qz_k 


0. 

THE   COXSTKUC  ri\E    I'KIXCII'LE   OF    THE    "CROSs"    RETINO- 
SKLV  METER, 

very  little  alteration  after  dynamic  skiametry  was  developed, 
although  the  addition  of  this  method  to  the  value  of  the  instru- 
ment has  been  found  too  great  to  be  estimated  by  figures. 

Binocular  action  was  also  an  original  feature  sought  for; 
then  simplicity ;  not  only  simplicity  of  construction  and  action, 
but  simplicity  of  operation.  Fig.  87  shows  the  mechanical 
arrangement  of  the  instrument  and  the  manner  of  the  adjust- 
ment of  its  lens  system. 


Tiiii  ki:ti.\()-ski.\.mi:ti:r  139 

Two  convex  ami  two  concave  cylindrical  lenses  of  seven 
diopters  each  are  mounted  in  cells  A  and  B,  and  A  and  B 
prime,  with  their  axes  at  rig^ht  ang^lcs  to  each  other,  each  lens 
being  slightly  inclined  from  the  perpendicular  on  its  meri- 
dional axis.  The  two  concave  lenses  are  stationary  while  the 
two  convex  ones  are  made  movable.  The  cells  of  the  latter 
slide  on  rods  H,  being  controlled  by  a  doubled  cord  C,  D. 
thirty-six  inches  in  length,  this  cord  passing  over  pulley  F. 
The  cord's  length  being  always  the  same,  an  operator  has 
merely  to  turn  his  hand  at  the  wrist  in  order  to  obtain  full 
control  of  the  refractive  changes  of  the  instrument.  The 
tilting  of  the  lenses  is  to  obviate  the  multiplicity  of  light  reflec- 
tions which  usually  hover  about  them  when  they  are  used  in 
an  upright  position  in  connection  with  skiametry  and  ophthal- 
moscopy. It  will  be  seen,  therefore,  that  cylindrical  lenses 
had  to  be  used ;  for  the  tilting  of  a  spherical  lens  at  any  angle 
produces  a  disturbance  in  its  refraction. 

In  the  use  of  convex  cylindrical  lenses  it  was  found  that 
by  placing  each  cell  on  a  sliding  block,  such  as  shown  by  K 
and  E,  and  by  then  using  a  hook  the  operator  could  unfasten 
them  while  working  at  a  distance  by  merely  moving  his  hand 
a  few  inches  to  one  side.  The  cylindrical  lenses  used  in  this 
way  could  also  be  made  to  serve  a  double  purpose :  for  when 
used  together  they  acted  as  a  single  spherical  lens  does,  but 
when  used  singly,  or  unlocked,  they  acted  as  simple  cylinders. 

Another  valuable  principle  was  also  discovered  and  made 
use  of.  It  was  found  that  as  the  plus  cylinders,  A  and  B 
prime,  were  moved  away  from  the  minus  cylinders.  A  and  B, 
any  object,  such  as  a  patient's  eye,  when  placed  close  up  to 
the  concave  lenses  would  be  magnified  by  the  convex  ones,  just 
as  though  a  plain  seven-diopter  convex  spherical  lens  had  been 
used,  this  magnification  taking  place  without  causing  the  least 
interference  in  the  refraction  of  the  four  lenses  while  they  were 


140 


THE    RETIXO-SKIAMETER 


being  used  as  a  lens  series  to  produce  mobility  of  action. 

Other  attachments  comprise  a  self-adjusting  brow  rest  to 
give  the  instrument  stability,  a  handle,  a  base  and  a  means  for 
separating  the  two  tubes.  The  large  holes  now  in  the  top  of 
the  tubes  of  the  latest  model  instrument  facilitate  the  cleaning 
of  the  lenses,  whose  cells  are  all  attached  to  blocks  on  the  slide 


Fig. 


E.XTERXAL    APPEAR.ANCE    OF    THE      CROSS       INSTRUMENT. 


rods.     The  minus  cylindrical  lenses  are  secured  in  position  by 
a  set  screw  in  the  block  holding  cell  B. 

Fig.  88  shows  the  instrument  com])letc,  with  its  auxiliary 
lens  disc  containing  three  sphcricals.  i.  e,  —  i.  —  3.  and  —  6.  D,. 


THE    RETINO-SKIAMETER 


141 


to  be  used  in  converting  the  instrument  from  the  "static"  to 
the  "dynamic"  method,  and  also  for  changing  the  total  refrac- 
tion from  plus  to  minus,  as  occasion  demands. 

The  pupillary  distance  is  made  easy  of  alteration  by  partially 
unscrewing  the  handle.  This  is  accomplished  by  holding  the 
instrument  in  the  manner  shown  in  Fig.  89.  The  index  finger 
of  the  left  hand  is  used  to  support  the  connecting  bars  between 
the  two  tubes,  or  sides,  and  enables  their  distance  to  be  altered 
by  merely  giving  the  handle  a  half  turn  or  so  with  the  right 

Fig.  89. 


SHOWING    HOW    TO    ALTER    PUPILLARY    DISTANCE. 


hand.     It  also  prevents  awkwardness  and  keeps  the  two  tubes 
always  parallel. 

The  six-pound  base  is  used  chiefly  as  a  holder  for  the  instru- 
ment when  not  in  use,  although  it  can  be  employed  on  a  table 
for  children  if  the  examiner  prefers.  The  light  weight  of  the 
device,  however,  which  is  only  nineteen  ounces,  enables  chil- 
dren even  as  young  as  four  years  of  age  to  hold  it  very  satis- 
factorily by  using  both  hands. 


142 


THE    RETIXO-SKIAMETEK 


The  manner  of  operation  is  not  difficult.  After  showing 
the  patient  how  to  hold  the  instrument  by  first  holding  it  be- 
fore his  own  eyes,  the  examiner  seats  himself  and  directs  the 
patient  to  look  at  the  largest  letter  on  the  card  of  the  fixation 
stand,  fifty-three  inches  distant,  if  the  examination  is  to  be  made 
by  the  dynamic  method,  or  over  his  shoulder  at  some  object 
twenty  feet  away,  if  the  static  method  is  to  be  employed. 

The  examiner  then  takes  one  of  the  looped  cords  in  his 
hand  and  pulls  it  just  sufficiently  to  keei)  it  taut  all  the  time, 
he  is  also  to  hold  it  in  such  a  manner  that  one  turn  of  tlie 

Fig.  90. 


SHOWING   MANNER  OF   USING   INSTRUMENT. 


wrist  will  give  him  the  full  refractive  power  of  the  instrument 
without  re-adjustment.  This  permits  of  a  range  from  zero  up 
to  six  diopters  of  plus  or  minus  spherical  or  cylindrical  lens 
quantity.  A  slight  pressure  of  the  little  finger  on  the  cord 
serves  to  keep  it  from  slip])ing.  And  if  the  little  finger  on  the 
mirror  hand  is  placed  on  the  thumb  of  the  cord-hand,  a  distance 


THE    RETIXO-SKIA.MliTliU 


M3 


equal  to  exactly  forty  inches  can  be  readily  secured.  The  double 
cord  being  just  thirty-six  inches  long,  the  arrangement  of 
the  hands  can  be  made  to  supply  the  other  four  inches.  Fig. 
90  illustrates  these  points. 

F[G.  91. 


SIIOWIXG    IXSTRUMKXT    IX    DUST-rROOF   CASE. 


With  one  hand  controlling  the  skiascope  and  the  other  hav- 
ing "a  trial  case  on  a  string,"  as  the  instrument  has  been  called, 
it  is  easy  to  see  that  shadow  and  refraction  can  be  made  to 
bear  a  relation  to  one  another  not  unlike  that  of  "bowing"  and 
"fingering"  in  the  playing  of  a  violin. 


144  THE    RETINO-SKIAMETER 

The  wooden  case  for  holding  the  instrument  when  not  in 
use  is  intended  to  keep  it  free  from  dust  and  dirt.  This  case 
can  be  made  in  a  variety  of  woods  and  can  be  finished  in  either 
a  plain  or  elaborate  manner.  The  cover  is  double-hinged  in 
such  a  way  that  the  removal  of  the  instrument  from  its  base  is 
made  very  easy.  Fig.  91  will  give  an  idea  of  the  construction 
and  appearance  of  the  case,  which  makes  a  handsome  ofifice 
ornament. 

As  a  very  liberal  criticism  of  other  instruments  and  de- 
vices has  been  made  in  these  pages,  it  would  seem  to  be  no 
more  than  fair  that  the  shortcomings  of  this  retinal-shadow 
measure  be  held  up  for  inspection  also.  It  is  an  instrument  that 
perhaps  requires  more  skill  to  use  than  do  any  of  the  other  de- 
vices to  which  attention  has  here  been  called.  It  needs  to  be 
kept  in  excellent  order  to  obtain  good'  service  from  it.  It  is 
somewhat  easy  to  get  out  of  adjustment,  and  is  very  sensitive 
to  abuse.  It  gets  dirty  readily,  and  needs  more  care  than  ordi- 
nary instruments  do.  It  is  troublesome  of  manufacture,  be- 
cause it  takes  so  much  skilled  labor  and  attention  to  detail  in 
the  selection  and  mounting  of  its  lenses  in  order  to  secure 
the  accuracy  essential  to  the  proper  performance  of  its  work. 
And  when  it  does  get  out  of  order  it  generally  has  to  be  sent 
to  the  factory  for  readjustment. 

There  is  one  disputed  point  of  excellence  in  the  operation 
of  the  instrument  to  which  especial  attention  is  invited.  This 
refers  to  the  apparent  production  of  mydriasis  by  magnifying 
the  pupil.  Enlargement  by  magnification  would  seemingly 
mean  an  increase  in  size  at  the  expense  of  definition.  Let  it 
be  considered,  then,  whether  this  is  really  so  or  not.  The 
word  "definition,"  as  used  in  optics,  means  the  po^ver  of  a 
lens  to  give  an  image  of  anything,  or  part,  so  as  to  clearly  dis- 
tinguish it  from  its  surroundings.  But  this  is  perhaps  mislead- 
ing, as  experienced  microscopists  say  that  a  lens  of  low  power 


THE    RETINO-SKIAMFITKR  I45 

often  works  better  than  one  of  high  power,  because  with  a  low- 
power  lens  a  better  general  idea  may  be  had  of  the  object,  even 
at  the  expense  of  size,  than  if  it  were  viewed  through  a  high- 
power  lens. 

It  might  be  reasoned  from  this,  therefore,  that  it  applied  to 
the  magnifying  of  the  retinal  shadow.  But  here  comes  the 
examiner's  own  vision  and  the  law  of  a  five-minute  angle  gov- 
erning its  acuity.  If  he  operated  at  a  much  nearer  point  than 
forty  inches,  any  increase  in  size  of  the  shadow  without  cor- 
respondmg  increase  in  the  substance  of  which  it  is  composed 
might  interfere  with  the  sharpness  of  demarcation  caused  by 
magnifying.  But  many  ocular  pupils  are  so  small  that  the 
retinal  shadow  gives  a  visual  angle  of  less  than  one-half,  per- 
haps, of  what  it  should  at  forty  inches  away,  and  so,  when  the 
pupil  is  magnified  to  several  times  its  original  size,  the  increase 
in  visual  angle  more  than  compensates  for  the  decrease  in 
sharpness  of  outline.  Hence  this  explanation  to  theoretically 
account  for  that  which  those  who  use  this  method  learn  to  be 
a  fact  from  actual  experience. 

A  simple  experiment  can  also  be  tried  which  will  serve  to 
emphasize  the  superiority  of  this  magnification  principle  as  here 
used.  If  a  patient  will  hold  a  convex  seven-diopter  spherical 
lens  two  inches  in  front  of  his  eye  and  let  an  examiner  compare 
the  sharpness  of  outline  of  the  union  between  the  iris  and 
the  sclerotic,  from  a  distance  of  forty  inches  away,  and  then 
determine  which  one  of  the  patient's  two  eyes  is  the  easiest  for 
him  to  see,  the  magnified  or  the  unmagnified  one.  the  differ- 
ence will  be  very  apparent,  and  the  larger  iris  will  seem  to 
lose  little  if  any  of  its  color  or  intensity  on  account  of  magnifi- 
cation. Fig.  92  illustrates  the  relative  size  between  an  average 
ocular  pupil  and  one  enlarged  three  diameters,  this  being  the 
maximum  magnification  by  means  of  the  author's  skiameter. 

The  working  distance  at  which  an  examiner  operates  will 


146 


THE    RETIXO-SKIA.METER 


of  course  affect  the  magnifying  power  of  whatever  lenses  may 
be  used,  but  as  a  rule  this  magnifying  principle  will  be  seen  to 


Fig.  9: 


o 


SHOWING  CO.Ml'ARAin'E  SIZE  OF  NORMAL  A.NJ)  .UAGMl-lED  PUPIL. 

take  care  of  itself,  for  in  high  degrees  of  hypermetropia,  where 
small  pupils  are  apt  to  be  found,  the  cnlargmcnt  caused  by  the 
lenses  in  the  instrument  will  usually  be  anii)lc  for  all  practical 
purposes. 


CHAPTER  X. 

Systematic  Ocular  Examinations  an  iTit  Aid  Dkrivhd 
FROM  ]\Iaking  Primi-:  Ri:coiiL)s. — Ri:sourcefulnes:  in 
Refraction  Work  and  the  Slxcessful  Examinatkj.v 
OF  THE  Eyes  of  Children,  Mutes  and  Illiterates. 

SYSTEMATIC  EXAMIXATIOXS.  As  one  of  the  pro- 
nounced aids  to  successful  examination-room  work,  a  brief 
reference  will  here  be  made  to  systematic  examinations  and  the 
practical  assistance  to  be  derived  from  carefully  recording 
them. 

The  great  value  of  this  troublesome  detail  can  not  be  em- 
phasized too  frequently,  for,  as  has  been  remarked  before,  ocu- 
lar skiametry  is  the  great  refractive  pilot  or  pathfinder,  and, 
therefore,  when  the  path  has  once  been  found  it  is  wise  to  keep 
it,  and  also  to  keep  track  of  its  various  windings. 

A  blank  form  should  be  used,  containing  properly  named 
spaces  wherein  entries  can  be  systematically  made,  so  that 
nothing  of  importance  may  be  overlooked  in  the  hurry  of  busy 
days.  This  blank  should  be  large  enough  to  contain  on  one 
sheet  a  complete  record  of  everything  pertaining  to  a  case. 

This  is  an  age  of  card  indexes,  but  the  makers  of  these 
valuable  time-savers  do  not  seem  to  appreciate  the  nccrls  of 
those  who  are  engaged  in  optometrical  work,  for  they  make 
their  cards  much  ioo  small  for  all  that  ought  to  be  entered  on 
them. 

In  the  author's  own  examination-room  work  he  has  adopted 


148 


SYSTEMATIC    EXAMINATIONS 


Fig.  93. 


Res.d 
Occu 

palion 

Slale 

of 

OD 

PHYSICAL    EXAMINATION. 

CO... 

0  s 

Total  > 
0    D 


VUlon 
Befora  uomeuon. 

0    D 

Test  Case: 

FUNCTIONAL    I 

0.  D 

XAMINATIO^ 

Cri. 

.... 

1    .n.r^cX;?So..oo. 

OD 

0    S 

0  s 

;o.  s. 

Esophona= 

E.ophoria  = 

Hyperphona= 

Presbyop.a 

Adduclion= 

Abduclion= 

Sursjmdn= 

1 

8pb., 

0  D 

Pratsnt  Clatses: 

Cyl..                           Ai* 

1                                  I 

Note: 

OS                 1                   1 

p_|0.  0                   1                     1 

10  s 

1                                  1 

Praicrlbed  ay 

FACIAL    MEASUREMENT; 

Temples 


■                                                                                      No.  of  Mounting 
For     0    D 

O.S 

S'le  ol  Len». 

Oelivet 

Retetred  by  . 


s.« 

o(  Lens 

Pnce  % 

FftP   % 

For 

0   0 

No.  Of  Mounting 

O.S 

Price.  $ 

ArT]ount  Paid,  $_ 


Tolal.  $_ 
_  Due.  $. 


£»  mr. 


SAMPLE  RECORD  BLANK. 


SYSTEMATIC    liXAMIXATIONS 


149 


a  plan  which,  so  far,  seems  to  answer  all  purposes  very  well 
indeed.  The  regulation  blank  used  is  shown  by  the  illustra- 
tion Fig.  93. 

The  back  of  it  is  plain.  This  sheet  is  of  very  heavy  linen 
paper,  and  is  about  six  inches  wide  by  nine  inches  long.  The 
records,  of  course,  are  kept  in  a  fireproof  safe,  and  to  have  them 
occupy  as  little  room  as  possible  a  number  of  heavy  tin  boxes, 
or  drawers,  are  provided  for  them,  these  boxes  being  kept  in 
compartments  so  as  to  avoid  confusion.  They  are  also  designed 
in  such  a  manner  as  to  facilitate  the  addition  or  removal  of  a 
single  sheet.     Fig.  94  illustrates  the  appearance  of  one  of  the 


boxes  when  filled  with  blanks. 


Each  sheet  carries  an  index 


Fig.  94. 


MANNER  OF  FILING  RECORDS. 

number,  and  a  very  large,  three-letter,  index  book  enables  the 
name  and  record  number  of  each  patient  to  be  easily  and  quickly 
found  The  blanks,  both  new  and  old.  which  are  made  use 
of  during  one  week,  serve  as  a  kind  of  day-book,  and  thus. 


150  SVSTliMATlC    EXAM  IXATIOXS 

in  the  end,  really  save  more  labor  from  a  bookkeeping  stand- 
point than  their  use  entails. 

'  To  describe  more  completely  the  uses  for  which  the  blank 
is  intended  it  can  be  stated  that  the  age  of  the  patient  may  be 
marked  in  cipher  so  that  inquisitive  persons  cannot  gratify 
idle  curiosity,  if  the  record  happens  to  be  left  carelessly  ex- 
posed. 

The  skiametric  examination  is  made  first,  then  follows  the 
keratometric  one,  that  is,  if  astigmatism  of  over  one  diopter 
is  present.  Vision  before  correction  is  then  recorded,  after 
which  corroboration  with  trial  lenses  is  made,  and  vision  after 
correction  is  noted.  If,  in  view  of  the  refractive  condition, 
satisfactory  vision  is  not  obtained  when  trial  lenses  are  used, 
then  the  ophthalmoscope  is  resorted  to  for  any  possible  clue 
which  it  might  be  able  to  give  as  to  the  reason  therefor.  Under 
the  term  "Fundus,"  a  pale  or  choked  disc,  or  any  other  abnor- 
mality may  be  entered.  Under  "Lens"  the  various  kinds  of 
cataracts  can  be  abbreviated  "Cort.  Cata.,"  signifying  the  cor- 
tical kind,  while  a  rough  pencil  sketch  marking  the  inside  of  a 
circle  will  often  serve  to  note,  at  a  future  exammation,  whether 
any  increase  or  descrease  in  number  and  density  of  striae,  or 
spicula.  has  occurred. 

Under  "Cornea"  the  word  "slight"  or  "dense"'  as  pertain- 
ing to  opacity,  frequently  makes  plain  the  reason  for  the  low 
percentage  of  acuity  which  the  figures  under  "V^ision  after 
correction"  indicate.  Presbyopia  is  always  recorded,  when 
present,  and  should  be  made  to  harmonize  as  far  as  possible 
with  Bonder's  rule,  approximating  one  diopter  for  ages  forty- 
five  to  forty-eight,  one  and  a  half  for  forty-eight  to  fifty,  two 
diopters  for  fifty  to  fifty-three,  two  and  a  half  for  fifty-three  to 
fifty-eight,  and  three  for  sixty  and  upwards.  T'^is  of  course  is 
in  addition  to  any  so-called  "errors"  wh'ch  may  ai?o  be  present. 

If  standard  visual  acuity  is  obtained,  and  the  examiner  feels 


SYSTEMATIC    EXAMIXATIONS  I5I 

certain  that  a  "latent"  error  is  not  present,  his  patient  having 
persevered  long  enough  with  glasses  to  have  pretty  well  broken 
up  any  old  ocular  muscle  habits,  and  yet  there  still  remains 
an  unsatisfactory  condition,  then  it  is  the  part  of  good  judg- 
ment to  record  the  action  of  the  extrinsic  muscles  in  the  space 
devoted  to  this  purpose,  and  resort  to  the  use  of  prisms.  All 
of  these  failing,  the  examiner  should  then  do  as  many  other 
professional  men  have  done  before  him  and  appeal  to  another 
"court."  or  advise  a  consultation. 

"Present  glasses,"  and  by  whom  prescribed,  is  always  a 
satisfactory  minute,  for  it  enables  an  examiner  to  keep  track  of 
the   mistakes   of   others,   even  if  he  cannot  of  his  own. 

"Facial  ^Measurements"  end  the  findings,  and  then  comes 
the  judgment  requisite  to  insure  success.  The  style  of  mount- 
ings, prices,  time  of  delivery,  by  whom  the  case  was  referred, 
and.  lastly,  the  examiner's  initials  or  name. 

For  the  purpose  of  explanation,  let  it  be  supposed  that  the 
examiner  is  fallible  and  docs  err  in  judgment;  the  patient  re- 
turns and  has  a  slight  change  made.  This  is  duly  recorded  on 
the  back  of  the  blank,  as  well  as  the  fact  that  no  charge  wasr 
made  for  this  change. 

In  the  course  of  a  few  months,  perhaps,  a  new  "O.  D."  is 
supplied,  which,  with  its  price,  is  also  recorded,  and  thus  the 
blank  will  last  for  years,  for  whenever  the  patient  calls  the 
blank  can  be  taken  into  the  examination-room  and  made  to 
serve  as  a  complete  "book  of  the  play." 

When  skiametric  examinations  are  recorded  they  seldom 
have  to  be  repented  in  return  cases,  for  only  subjective  tests 
and  variations  in  judgment  being  all  that  is  ordinarily  required. 

Therefore,  when  it  is  stated  that  systematic  ocular  examina- 
tion records  are  very  valuable,  from  a  practical  standpoint,  this 
statement  might  go  further  and  class  them  along  with  examina- 
tion rooms  and  claim  both  as  absolutely  necessary  in  this  day 


152  RESOURCEFULNESS 

and  age,  when  a  pronounced  success  in  optometry  is  only 
achieved  by  paying  strict  attention  to  every  detail  of  method, 
place  and  device. 

RESOURCEFULNESS.  Resourcefulness  is  another  im- 
portant quality  for  optometricians  to  possess.  For  when  to 
rely  on  a  patient's  "Yes"  or  "No,"  and  when  not  to,  requires 
no  small  amount  of  knowledge  of  human  nature  as  well  as 
ability  as  a  cross-questioner.  Of  course  ocular  skiametry  and 
other  objective  means  place  an  examiner  in  a  position  largely 
independent  of  a  patient's  answers  or  intelligence,  yet  it  is 
always  a  source  of  satisfaction  to  have  one  method  corroborate 
another,  since  there  are  many  ways  in  which  to  be  wrong,  and, 
as  a  rule,  only  one  way  in  which  to  be  exactly  right.  With 
so-called  "regular"  conditions  skiascopists  are  likely  to  have 
little  trouble,  but  the  "irregular"  kind  frequently  call  for  con- 
siderable versatility  on  the  part  of  an  examiner  in  order  to  ex- 
tricate himself  from  a  refractive  corner,  so  to  speak. 

To  illustrate  this,  a  case  of  nystagmus  once  presented  itself 
which  had  been  seen  by  a  half-dozen  able  specialists.  The  age 
of  the  patient  was  twenty  years,  and  the  glasses  in  use  were 
one  diopter  concave  sphericals  for  both  eyes,  which  gave  an 
acuity  of  vision  equal  to  about  ten  two-hundredths. 

The  use  of  the  skiascope  showed  the  presence  of  myopic 
astigmatism,  linth  the  rule,  but  the  spasmodic  action  of  the  mus- 
cles precluded  the  ascertainment  of  the  amount.  By  recourse 
to  the  keratometer  and  engaging  the  patient  in  a  long  conver- 
sation regarding  Fchool  work,  in  order  to  quiet  the  so^sm.  it 
became  possible  to  locate  a  corneal  mal-curvature  of  about  five 
diopters.  The  use  of  the  mirror  again  showed  the  error  to  be 
free  from  hypermetropia,  and  glasses  of  four  diopters  concave 
cylindrical  at  axis  180,  after  thirty  days'  use,  increased  vision 
up  tf)  a  little  better  than  twenty  one-hundredths. 

When  it  is  borne  in  mind  that  this  case  had  been  under  care- 


RESOURCEFULNESS  1 53 

ful  observation  for  twelve  years,  and  that  no  expense  had  been 
spared  in  consulting  the  most  prominent  specialists,  the  joy  of 
the  patient  over  the  results  achieved  can  be  readily  imagined. 

A  hap-hazard  trial  of  test  lenses  might  have  resulted,  in 
this  case,  in  a  clue,  which  could  have  been  followed  up  satis- 
factorily, perhaps,  but  this  less  certain  method  frequently  leads 
an  examiner  astray  through  the  patient's  failure  to  appreciate 
and  give  an  encouraging  answer  to  a  partial  correction. 

In  the  refractive  examination  of  the  eyes  of  children,  mutes 
^and  illiterates,  ocular  skiametry  offers  about  the  only  truly 
reliable  means  for  independently  determining  the  kind  and 
strength  of  proper  glasses. 

In  this  same  category  might  be  included  those  persons  who 
are  only  partially  deaf  and  who  fail  to  respond  to  all  questions 
asked  them.  Also  those  persons  who  do  not  speak  the  same 
language  the  examiner  does,  and  careless  persons  who  some- 
times prefer  to  joke  and  unconsciously  cause  an  examiner  to 
become  careless  himself.  Then  there  are  the  ultra-careful  per- 
sons, whose  answers  are  about  as  misleading  as  if  they  were 
careless. 

All  of  these  classes,  and  many  others,  tend  to  show  the  value 
of  ocular  skiametry,  for  success  must  be  achieved  no  matter 
what  the  excuse  or  obstacle  may  be,  as  any  examiner,  no  mat- 
ter where  located,  can  ill  afford  to  endanger  his  reputation  by 
doing  poor  work. 


CHAPTER  XI. 

Illustrative  Cases,  Showing  the  Comparative  A'alue  of 
Static  and  Dynamic  Skiametrv  ix  \'arious  Patients 
OF  Different  Ages,  Occupations  and  Apparent  Phys- 
ical Condition. 

ILLUSTRATIVE  CASES.  The  expression  "Figures  talk" 
is  especially  applicable  in  describing  the  relative  merits  of  static 
and  dynamic  methods  in  practising  ocular  skiametry.  This 
chapter,  therefore,  will  be  devoted  largely  to  descriptions  of 
various  cases  for  the  purpose  of  emphasizing  points  already 
alluded  to,  and  of  incidentally  calling  attention  to  the  influence 
of  occupation  and  the  importance  which  attaches  to  the  condi- 
tion of  the  patient's  general  health.  All  the  examinations 
referred  to  here  were  made  without  the  aid  of  cycloplegics,  con- 
sequently the  static  test  mentioned  is  the  non-toxic  one. 

It  will  be  observed  in  viewing  these  cases  that  the  measure- 
ments founfl  and  the  formulas  given  are  not  always  in  accord. 
This  is  due  to  the  fact  that  optometric  findings,  as  a 
rule,  must  be  tempered  with  judgment,  for  old  examiners 
know  from  experience  that  their  "come-back"  cases  show  more 
faults  in  this  respect  than  to  mistakes  due  to  variations  in 
subsequent  measurements. 


ILLUSTRATIVE   CASES  l.SS 


CASE  A. 


:Master  S.,  age  7.     In  school.     Health,  seemingly  good; 
O.   S.   shows   slight   convergent  squint. 

Vision  =  O.  D.  20/20.    O.  S.  20/100. 

Static  test  =:  O.  D.  +  2.50  D.  S.  O.  S.  +  3-    D-  S. 

Dynamic  test  at  forty  inches  =  O.  U.  4"  3-50  D.  S. 

Dynamic  test  at  twenty  inches  =  O.  U.  +  4.  D.  S. 

Dynamic  test  at  thirteen  inches  =  Unsatisfactory. 

Trial  case  test  =  O.  D.  +  2.  D.  S.    O.  S.  +  3.  D.  S. 

Vision  =  O.  D.  20/20.    O.  D.  20/80. 

Formula  given  =  O.  U.  -f  3.  D.  S. 

For  constant  use,  with  instructions  to  return  in  one  year. 

Two  years  later.  O.  U.  +  3.50  D.  S.  was  readily  accepted.    Xo 

squint. 

\'ision  r=  O.  D.  20/20.     O.  S.  20/40. 

CASE  B. 

Master  W.,  age  ir.     In  school.     Health  not  rugged.     In- 
ability to  see  blackboard. 

Vision  =  O.  U.  20/100.    Static  test  =  O.  U.  —  i.  D.  S. 

Dynamic  test  at  13  inches  =  about,  O.  I'.  —  0.50  D.  S. 

Trial  case  test  =  O.  U.  —  T.25  D.  S. 

Vision  =  20/20  in  both  eyes. 

Formula  given  =  O.  U.  —  0.50  D.  S. 

For  constant  use.    Instructed  to  return  in  three  months. 

Six  months  later  vision  O.  U.  =  20/20  with 

O.D.  — 0.75— D.  S. 

O.  S.  —  0.75  — D.  S. 


^5^  ILLUSTRATIVE   CASES 

CASE    C. 

Miss  N.,  age  i6.    In  school.    Health  fair.    Headaches. 

Vision  =  O.  U.  20/30. 

Static  test  =  O.  U.  +  0.50  D.  C.  90°. 

Dynamic  test  at  20  inches,  same. 

Trial  case  test  =  O.  U.  —  0.50  D.  C.  180°. 

Vision  =  20/20  in  both  eyes. 

Formula  given  =  0.11.4-  0.50  D.  C.  90°. 

Vision  =  "misty." 

To  use  at  study,  and  oftener  if  more  comfortable. 

Later  on  she  reported  •'no  headaches,"  and  vision  was  found 

to  be  20/20. 

CASE  D. 

Mr.  G.,  age  20.     In  college.    Reports  his  health  good.     No 
discomfort,  but  "can't  see  at  a  distance." 

Vision  =  O.  U.  20/200.    Static  test  =  O.  U.  —  2.50  D.  S. 

Dynamic  test  at  40  inches  =  O.  U.  —  2.  D.  S. 

At  20  inches,  about  the  same. 

Trial  case  test  =  O.  U.  —  2.75  D.  S. 

Vision  =r  20/20  in  both  eyes. 

Formula  given  =  O.  U.  —  2.  D.  S. 

For  constant  use. 

Instructed  to  return  if  he  had  any  further  trouble.     No  report. 

CASE  E. 

Mr.  S.,  age  24.     Mechanic.     Something  of  an  athlete;  com- 
plains of  headache. 

Vision  O.  U.  =  20/20. 

Static  test  O.  U.  =  +  1.25  D.  S.  Z  +  0.25  D.  C.  90°. 


ILLUSTRATIVE  CASES  I57 

Dynamic  test  at  13  inches  = 

O.  U.  =  +  2.  D.  S.  C  +  0.25  D.  C.  90°. 

Trial  case  test  O.  U.  =  +  i.  D.  S.  3  +  0.37  D.  C.  90°. 

Vision  =  20/20. 

Formula  given  O.  U.  =  +  i.  D.  S.  ~  +  0.25  D.  C.  90'. 

Advised  to  return  in  one  year,  which  he  did,  and  was  given 

O.  U.  =  +  1.50  D.  S.  C  +  0.25  D.  C.  90°.    Was  then  advised 

to  return  again  in  two  years  more. 

CASE  F. 

Miss  F.,  age  26.     Seamstress.     General  health  not  good. 
"Weak  eyes." 

Vision  =  O.  D.  20/40.    O.  S.  20/80.  . 
Has  been  using  O.  U.-|-  i  D.  S. 
Static  Test  = 

O.  D.  4-  0.50  D.  S.  C  +  2.  D.  C.  90°. 
O.  S.  +  2.  D.  S.  C  +  2.  D.  C.  105°. 
Dynamic  test  at  40  inches  = 
O.  D.  +  I.  D.  S.  C  +  2.  D.  C.  90°. 
O.  D.  4-  3.  D.  S.  C  +  2.  D.  C.  105°. 
Dynamic  test  at  20  inches  = 
O.  D.  +  1.50  D.  S.  C  +  2.  D.  C.  90°. 
O.  S.  +  3-  D.  S.  C  +  2.  D.  C.  105°. 
Dynamic  test  at  13  inches  ^  Not  satisfactory. 
Keratometer  =  O.  D.  2.  D.  90°.    O.  S.  2.  D.  1 10°. 
(Note  change  in  axis  of  O.  S.) 
Trial  case  test  = 

O.  D.  +  0.25  D.  S.  C  +  2.  D.  C.  90^ 
O.  S.  +  1.50  D.  S.  C  +  2.  D.  C.  105". 
A'ision  =:  O.  D.  20/20.     O.  S.  20/40. 
Formula  given  = 

O.  D.  +  1.    D.  S.  C  +  2.  D.  C.  90°. 
O.  S.  +  2.50  D.  S.  C  +  2.  D.  C.  105°. 


158  ILLUSTRATIVE   CASES 

For  constant  use,  with  instructions  to  "never  mind  if  distant 
objects  are  a  trifle  blurred  for  a  few  weeks."  Returned  in  a 
week  with  a  history  of  occasional  discomfort.  Gave  advice  to 
persevere.  Returned  in  four  months  with  a  broken  lens  and 
wanted  a  new  one  "immediately."  Glasses  were  very  satis- 
factory.   \'ision  =  O.  D.  20/20.     O.  S.  20/40. 

CASE  G. 

Mr.  C,  ai::::e  29.  Bookkeeper.  Reports  health  good  when 
not  working  too  hard.  Eyes  and  head  "feel  bad"  afternoons. 
Has  been  wearing  glasses  = 

O.  D.  —  I.  D.  S.  Z  +  2.  D.  C.  75°. 

O.  S.  —  I.  D.  S.  C  +  150  D.  C.  105°. 

For  three  years. 

Vision  with  present  glasses  =  O.  U.  20/30.     Fundus  reflex  is 

very  poor. 

Keratomcter  shows  =  O.  D.  2.  D.  axis  75°.     O.  S.  2.  D.  axis 

105°. 

Dynamic  test  at  thirteen   inches,   with  trial  lenses,   show  the 

myopic  quantity  to  be  only  0.50  D.  in  both  eyes. 

Trial  case  test  = 

O.  D.  —  I.  D.  S.  C  +  2.  D.  C.  75°. 

O.  S.  —  I.  D.  S.  C  +  2.  D.  C.  105°. 

Vision  =  20/30. 

Formula  given  = 

O.  D.  —  0.50  D.  S.  C  +  2.  D.  C.  75°. 

O.  S.  —  0.50  D.  S.  ~  4-  2.  D.  C.  I05^ 

Repr)rt  received  in  two  weeks  was  "All  r'ght  n  av." 

CA.SE  TT. 

Mr.  R..  age  34.  Grf)cer.  History  of  health  unsatisfactory. 
Vision  poor  for  past  few  months. 


ILLUSTRATIVE   CASES  159 

Present  vision  =  20/80  in  both  eyes. 

Static  test  = 

O.  D.  +  I.  D.  S.  I  +  0.50  D.  C.  180°. 

O.  S.  4-  I.  D.  S.  Z  +  0.50  D.  C.  180°. 

Dynamic  test,  about  the  same.     Trial  case,  about  the  same. 

Optical  correction  no  material  aid  to  vision.  Ophthalmo- 
scope shows  pale  discs.  Close  questioning  leads  to  conclusion 
that  it  is  a  probable  case  of  nicotine  poisoning  due  to  im- 
moderate smoking  and  moderate  use  of  alcohol.  Gave  no 
glasses.    Advised  to  consult  an  oculist  first. 

CASE  J. 

Miss  B..  age  38.  Stenographer.  Says  health  is  good  ex- 
cept for  headaches. 

\-ision  =  O.  U.  20/20. 

Static  test  =  O.  U.  +  0.50  D.  S. 

Dynamic  test  at  40  inches  =  O.  U.  +  0.75  D.  S. 

Trial  case  test  =  O.  U.  +  0.25  D.  S. 

\lsion  =  20/20  trifle  "hazy." 

Formula  given  =  O.  V.  +  0.50  D.  S. 

For  reading  and  near  work.    Good  report. 

CASE  K. 
Mrs.  A.,  age  41.    Has  household  cares  only.    General  health 
none  too  good.     Complains  of  inability  to  see  to  thread  her 
needle  and  do  fancy  work.    No  headaches. 
Vision  =  O.  U.  20/20.    Dynamic  test  at  40  inches  r=  O.  U  -+- 
i.D.S.    Trial  cast  test  =  O.  U.  +  0.75  D- -^-    Vision  =  20/20. 
Formula  given  =  +  i.  D.  S.  for  both  eyes,  with  instructions 
to  use  for  near  work. 
No  report. 


l6o  ILLUSTRATIVE   CASES 

CASE  L. 

Mrs.  L.,  age  46.  Housekeeper.  Health  appears  good.  Dif- 
ficulty in  reading.     Xo  headache. 

Vision  =  O.  U.  20/20.    Static  test  r=  O.  U.  +  0.25  D.  C.  90°. 
Dynamic  test,  about  the  same.    Trial  case,  about  the  same. 
Presbyopia  =  i.  D. 

Formula  given   =   C).   L'.   +   i.  D.   S.    Z    +  0.25  D.   C.   for 
reading  etc.     Xo  report. 

CASE  M. 

Mr.  D.,  age  52.  Court  stenographer.  Health  seemingly 
good.  X'ever  has  had  any  glasses  that  proved  quite  satis- 
factory. 

Vision  =  O.  U.  20/80.    Dynamic  test  at  50  inches  = 

O.  D.  +  1.25  D.  S.  C  +  0.25  D.  C.  135°. 

O.  S.  +  1.25  D.  S.  C  +  0.25  D.  C.  90°. 

Trial  case  test,  the  same.    \'ision  =  O.  U.  20/30. 

Presbyopia  =  2.25  D.     Gave  bi-focals. 

Reported  in  sixty  days  that  vision  was  good  but  glasses  did 

not  seem  quite  right.    "Guessed"  he  was  working  too  hard. 

Re-cxamination  by  dynamic  test  at  30  inches  = 

O.  D.  -f  1.50  D.  S.  C  +  0.25  D.  C.  120°. 

O.  S.  +  1.75  D.  S.  C  -f  0.25  D.  C.  80°. 

Trial  case,  the  same.    \'ision  :=  O.  U.  20/20. 

Presbyopia  =  2.  D. 

Reported  in  six  months  "O.  K.  now.  'twas  the  glasses  after  all." 


ILLUSTRATIVE   CASES  l6l 

CASE  X. 

Mr.  O'B.,  age  55.    Driver.    Health  good.    "Can't  see." 

Vision  =  O.  U.  20/80  .    Static  test  =  O.  U.  +  1.50  D.  S. 

Presbyopia  =:  2.50  D. 

Trial  case  test  =  O.  U.  +  1.50  D.  S. 

Vision  =  20/20. 

Formula  given  =  O.  U.  -f  4-  D.  S.  for  reading.    No  report. 

CASE  O. 

Mr.  E..  age  59.  Tailor  and  cutter.  Health  good.  Working 
distance  is  about  twenty  inches  away.  Present  glasses  are  -J-  3. 
D.  S  for  both  eyes,  and  are  not  very  satisfactory. 

Vision  =  O.  D.  20/30.    O.  S.  20/100. 

Static  test  = 

O.D. +  0.50D.  S. 

O.  S. +  1.      D.  S.  +  i.D.C.  180°. 

Keratometer  shows   no  corneal   nial-curvature   in   either  eye. 

Trial  case  test,  same  as  static  test.     X'ision  =  O.  D.  20/20. 

O.  S.  20/40. 

Presbyopia  at  working  distance  =  2.  D. 

Presbyopia  at  reading  di.stance  =  2.75  D. 

Formula  for  working  glasses  = 

O.D. +  2.50  D.S. 

O.S.  +3.      D.  S.  +  I.D.C.I8o^ 

Formula  for  reading  glasses  = 

O.  D.  +  3.25  D-  ^'^• 

O.  S.  +  3.50  D.  S.  +  I.  D.  C.  180°. 

Instructed  to  return  if  not  satisfactory.    No  report. 


I 62  ILLUSTRATIVE   CASES 

CASE    P. 

Mrs.  M..  age  62.  (Occupation  (?).  Health  (?).  Looks 
well. 

\'ision  less  than  20/200  in  both  eyes. 

Static  test :  First  attempt,  no  retinal  reflex.  Without  ski- 
ameter the  mirror  shows  small  pupils  and  slow  plus  movement. 
With  skiameter,  lenses  being  set  to  register  +  3.,  enlarged 
pupil  shows  better  movement  and  reveals  long,  narrow,  spike- 
like patches.  Error  about  -|-  4.  D.  S.  in  both  eyes. 
Trial  case  test  =  ( ).  L".  +  3.50  D.  S. 
Vision  =  ( ).  D.  20/40.    O.  S.  20/60. 

Presbyopia  =  3.  D.  Ophthalmoscope  shows  slight  cortical 
cataracts. 

Gave  formulas :  Distance  =  ( ).  L'.  +  3.50  D.  S. 
Reading  =  ( ).  U.  +  6.50  D.  S. 

With  instructions  to  be  sure  and  have  a  strong  light  coming 
over  shoulder  wilien  reading  or  sewing.  Sent  letter  to  family 
physician. 

CASE  Q. 

Mr.  McE.,  age  67.  Health  fair.  Retired.  Now  using 
glasses  +  4  D.  S.  for  reading;  wonders  if  they  can  be  inn 
proved. 

Vision  =  O.  U.  20/80,  which  is  improved  by  partially  closing 
the  eyelids.  Static  test  =  O.  U.  -f  i.  D.  S.  Trial  case  test  the 
same.  Vision  =  O.  U.  20/30.  Presbyopia  =  -f-  3.  D.  S. 
Formula  for  distance  =  +  i.  D.  S.  Advised  to  continue  with 
present  reading  glasses  and  to  increase  his  illumination  when 
using  his  eyes  for  near  purposes.    No  report. 


JLLISIRAT1\1-:   CASIiS  163 

Note. — According  to  Donders  the  near  point  of  distinct  vision 
in  an  emmetropic  eye  is  as  follows  : 

At  10  years  of  age  it  is  2^4   inches  away. 

"  20      4 

"  30      5'^       *■ 

"  40      '     "    "     9 

"  50      '    "   16 

"  60      40  "  " 

To  illustrate  in  fuller  detail  the  workings  of  dynamic  ski- 
ametry,  let  the  following  case  be  considered:  Mr.  H.,  age  25. 
Contractor's  timekeeper.  Leads  outdoor  life.  General  health 
excellent.     Complains  of  occasional  headache. 

Vision  =  O.  U.  20/20.    Static  test  =  O.  L'.  +  1.25  D.  $. 
Dynamic  test  at  13  inches  =:  O.  U.  +  2.  D.  S. 
Trial  case  test  =  O.  U.  +  i.  D.  S.    \'ision  =  20/20. 

"By  reference  to  Fig.  75,  it  will  be  seen  that  when  his  accom- 
modation and  convergence  each  receives  the  standard  amount 
of  innervation,  as  shown  in  Fig.  74.  the  convergence  will  be 
greater  than  the  accommodation,  and  binocular  confusion  will 
result,  thus  giving  rise  to  esophoria  unless  the  innervation  is 
altered  in  some  way  so  as  to  produce  the  condition  called  for 
by  Fig.  76,  where  the  innervation  for  accommodation  is  in 
excess  of  that  for  convergence.  .\  test  of  his  extrinsic  mus- 
cles, however,  shows  a  manifest  orthophoria  without  glasses. 

Now  what  are  the  deductions  that  may  be  drawn  from 
this  case?  Twenty-five  years  of  daily  use  of  the  eyes  without 
glasses  has  established  a  habit  of  adjustment  whereby  the  stand- 
ard relation  between  accommodation  and  convergence  has  been 
replaced  by  a  condition  in  which  convergence  has  given  way  a 
little,  otherwise  esophoria  or  a  mild  form  of  convergent  strabis- 
mus would  have  manifested  itself. 


164  ILLUSTRATIVE   CASES 

The  non-toxico-static  test  shows  a  reversal  of  the  shadow 
when  one  and  a  quarter  diopters  of  convex  lens  power  are 
added.  This  is  in  addition,  of  course,  to  the  quantity  necessary 
to  create  the  skiametric,  or  working,  refraction  when  trial-case 
lenses  are  used.  Thus  proving  that  habit  has  not  mastered  all 
of  the  error,  as  the  accommodation  readily  accepts  partial  as- 
sistance and  relaxes  its  miuscle  tension  as  much  as  five-eighths 
of  the  full  ametropia  present.  The  remaining  three-eighths 
of  the  total  error  can  be  called  latent,  but  in  reality  it  represents 
a  tonic  spasm,  a  knowledge  of  the  presence  of  which  materially 
aids  an  examiner  in  the  formation  of  his  judgment  and  in  the 
advice  and  prognosis  he  gives  a  patient. 

To  determine  the  amount  of  tonic  spasm  present  in  a  case 
such  as  the  one  under  consideration,  it  will  be  necessary  to 
resort  to  either  the  toxico-static  test  or  else  to  the  dynamic  one, 
and  as  it  is  usually  wise  to  go  in  the  direction  of  least  trouble, 
especially  when  this  direction  is  the  best  one,  recourse,  there- 
fore, is  to  be  had  to  the  test  that  calls  for  a  pronounced  exer- 
tion of  the  patient's  accommodation. 

An  emmetrope  25  years  of  age  is  supposed  to  have  about 
eight  diopters  of  amplitude  of  accommodation.  The  nearest 
point  of  distinct  vision  is  then  five  inches  away  from  the  eyes. 
A  dynamic  test  made  at  thirteen  inches  calls  for  an  accommo- 
dation equal  to  three  diopters.  The  i)atient's  error  being  two 
diopters,  it  follows  that  a  total  ocular  muscle  exertion  equal  to 
five  diopters  is  necessary  in  order  to  enable  the  patient  to  dis- 
tinctly read  small  letters  on  a  card  whose  distance  away  is  the 
same  as  that  of  the  examiner's  mirror. 

More  than  five  diopters  of  accommodative  effort  can.  of 
course,  be  exerted  by  the  patient  in  this  case.  Yet  this  amount 
will  generally  be  found,  in  like  cases,  quite  sufficient  to  break 
up  any  tonic  spasm,  or  habit  of  muscle  exertion,  that  mav  have 
been  formed.     If  the  examiner  is  supplied  with  a  device  which 


ILMSlKAl  1\  1-.    CASES  1^5 

permits  of  mobile  lens  action,  he  has  only  to  slide  in,  so  to 
speak,  the  required  refractive  assistance  necessary  to  cause  the 
accommodation  to  relax  until  it  has  assumed  its  standard  rela- 
tionship with  convergence.  When  this  test  is  made  at  thirteen 
inches,  as  before  stated,  the  accommodation  equals  three  diop- 
ters. Therefore,  five  diopters  less  three  diopters  leaves  two 
diopters  as  the  lens  quantity  that  will  reverse  the  shadow  by 
the  dynamic  test  under  these  conditions. 

If  the  test  had  been  made  at  twenty  inches,  then  four  diop- 
ters would  represent  the  total  muscle  effort  called  for.  If  at 
ten  inches,  then  six  diopters  would  be  the  full  accommodation 
needed.  The  difference  between  these  amounts  and  that 
required  to  maintain  normal  relationship  between  accommoda- 
tion and  convergence  at  whatever  distance  the  test  is  made  will 
show  at  once  in  the  lens  quantity  required  to  reverse  the 
shadow,  provided  an  adequate  lens  system  is  used,  and  pro- 
vided, also,  that  the  eyes  are  examined  in  a  semi-binocular  man- 
ner—namely, first  one  eye  and  then  the  other,  alternating  fre- 
quentlv— so  as  to  insure  an  equality  of  visual  fixation. 

One  point  which  seems  to  puzzle  many  examiners  who  take 
an  interest  in  making  theory  substantiate  practice  is  to  under- 
stand why  an  emmetropic  eye  when  under  an  accommodative 
tension  of  three  diopters  at  thirteen  inches,  will  not  relax  to 
two  diopters  when  one  diopter  of  assistance  is  offered  it.  The 
answer  to  this  query  probably  lies  in  a  better  understanding 
of  muscular  co-ordination  and  innervation,  for.  as  stated  in 
earlier  chapters,  the  eyes  of  a  healthy  person,  free  from  intoxi- 
cation cannot  converge  without  accommodating,  nor  can  they 
accommodate  without  converging.  And  this  co-ordinate 
relationship  will  respond  to  approximate  standards  unless  long- 
standing abnormal  requirements  have  induced  irregular  habits. 
In  this  latter  case  refractive  measurements  must  be  taken  m 
such  a  manner  as  to  estimate  the  real  influence  of  these  habits  by 


l66  II.I.ISTKATUK   CASES 

making  the  eyes  work,  for  the  time  being-,  in  a  manner  as  far 
removed  from  old  beaten  paths  as  possible. 

Now  another  case  will  be  cited  in  order  that  the  details  of 
skiametric  procedure  may  be  accentuated. 

Mr.  (  ?),  age  thirty-five,  occupation  watchmaker.  Has  been 
studying  optics  for  two  years.  States  that  he  has  fitted  himself 
with  O.  U.  —  0.50  D.  S.  —  0.75  D.  C.  axis  180°.  that  his 
vision  without  glasses  is  O.  D.  =  15/30  O.  S.  =  15/20.  and 
that  he  has  four  degrees  of  esophoria. 

As  the  above  information,  excepting  the  age,  is  supplied 
after  the  examination  is  finished,  the  examiner,  of  course,  pro- 
ceeds in  the  usual  manner  to  seat  the  patient  and  place  a  skiam- 
eter in  his  hand.  He  then  directs  the  patient  to  look  at  the  let- 
ters on  a  fixation  card  situated  fifty-three  inches  distant.  Be- 
ginning the  examination  at  forty  inches,  the  examiner  finds 
that  in  the  right  eye  there  is  a  fairly  distinct  edge  to  the  shadow 
and  that  it  points  a  little  to  the  left  of  the  vertical  meridian. 
Adding  convex  lens  quantity,  it  is  found  that  one  diopter  is 
needed  to  reverse  the  shadow  in  the  horizontal  meridian,  and 
that  in  the  vertical,  with  no  lens  power  added,  the  motion  is 
a  trifle  against  the  mirror.  With  the  patient  still  looking  at  the 
fixation  card,  fifty-three  inches  away,  the  examiner  finds  that 
he  must  advance  his  mirror  ten  or  twelve  inches  nearer  to  his 
patient  before  he  obtains  a  reversal  of  the  shadow  in  this  meri- 
dian. So  he  notes  on  his  examination  blank  "( ).  D.  —  0.25 
D.  S.  I   +   I.  D.  C.  ax.   105."' 

In  the  left  eye  the  horizontal  motion  is  reversed  with  a  half- 
diopter  convex  lens  quantity.  In  the  vertical  meridian  there 
is  just  a  suggestion  of  a  motion  icith  the  mirror,  when  the  ex- 
aminer is  forty  inches  away.  Adding  even  a  slight  convex  lens 
power  stops  it.  The  axis  seems  to  be  about  fifteen  degrees 
to  the  temporal  side  of  the  head.  The  examiner  notes  "O.  S. 
-|-  0.50  D.  C.  ax.  y=,."     With  the  skiameter  removed  the  exam- 


ILLUSTRATIVE   CASES  IbJ 

iner  see  a  brighter  reflex  and  a  more  pronounced  astigmatic 
straight  edge,  and  with  the  patient  looking  over  the  examiner's 
shoulder  at  test  types  twenty  feet  away  the  shadow  moves  tit7/i 
the  mirror  in  all  meridians  in  both  eyes.  With  the  patient  look- 
ing at  the  fixation  card  again  the  motion  in  the  vertical  meridian 
of  the  right  eye  can  not  be  reversed  until  the  fixation  stand 
has  been  moved  eighty  inches  away.  Corroborating  with  the 
trial  case,  it  is  found  that  vision  O.  U.  20/20,  a  trifle  "misty." 
can  be  secured  with  O.  D.  —  0.50  D.  S.  ~  +  0.75  D.  C.  axis 
105  and  O.  S.  +  0.25  D.  C.  axis  75.  Patient  reads  well  with 
this  correction,  and " — 0.50  D.  S.  or  +  0.50  D.  S.  added  in  a 
binocular  way  ofifers  no  aid.  Corroborating  skiametrically 
again  with  the  full  correction  on,  it  is  found  that  a  cpiarter- 
diopter  convex  lens  quantity  reverses  the  shadow  in  all  meri- 
dians when  the  patient  looks  at  the  brow  card  on  the  examiner's 
mirror,  no  matter  whether  its  distance  be  twenty  or  sixty  inches 
away.  With  the  quarter-diopter  convex  lens  power  removed, 
the  shadow  sho\ys  a  suggestion  of  a  movement  icith  the  mirror 
at  the  same  distances  of  twenty  and  sixty  inches  away. 

The  above  formula  is  then  subjectively  confirmed  and  the 
patient  is  instructed  to  wear  the  glasses  as  much  as  possible 
and  to  report  in  a  month.  All  the  tests  taken  together  occupy 
not  over  ten  or  twelve  minutes  of  time. 

In  analyzing  this  case  the  occupation  of  the  patient  is  l)orne 
in  mind  as  one  calling  for  considerable  accommodative  adjust- 
ment. Then  the  previous  wearing  of  concave  lenses  is  perhaps 
partly  responsible  for  the  four  degrees  of  esophoria  complained 
of.  for,  with  these  glasses  on,  one  end  of  the  astigmatic  interval 
in  the  left  eye  calls  for  one  and  a  half  diopters  of  accommoda- 
tion, whidi  in  turn  calls  for  two  and  a  quarter  degrees  of  con- 
vergence in  order  to  maintain  standard  co-ordination.  And  this 
for  one  eye  only. 

The  age  of  the  patient,  the  habit  of  excessive  convergence 


l68  ILLUSTRATUE   CASES 

due  to  occupation,  also  the  habit  of  accommodation  aggravated 
by  the  occasional  use  of  glasses  calling  for  increased  ciliary 
effort,  are  all  factors  to  be  considered  by  an  examiner,  espe- 
cially if  his  patient  returns  in  a  day  or  two  and  complains  of  a 
"thin  fog,"  etc. 

The  temptation  for  a  rcfractionist  to  advise  the  immediate 
use  of  lenses  which  he  feels  sure  represent  the  full  correction  of 
his  patient's  ocular  error  is  very  strong  indeed,  and  if  he  has  an 
intelligent  patient  to  reason  with  this  judgment  is  often  correct. 
But  if  his  patient  happens  to  be  of  the  timid  kind,  or  one  who 
thinks  the  acuity  of  vision  to  be  had  after  one  day's  use  of 
glasses  is  the  only  thing  to  judge  their  merits  by,  then  it  is 
wise  to  "make  two  bites  of  a  cherry,"  and  indulge  the  patient's 
own  notions  by  giving  a  temporary  correction  slightly  over  or 
under  that  which  is  really  indicated,  and  which  will  eventually 
have  to  be  given. 

It  is  cases  such  as  these  that  render  the  science  of  physio- 
logical optics  inexact,  for  an  examiner  must  always  remember 
that  attached  to  every  pair  of  eyes  is  a  different  individual  with 
a  different  body,  a  different  occupation,  different  habits  and 
different  ideas  as  to  different  things,  and  so  each  patient  requires 
different  judgment  and  different  explanations  and  encourage- 
ments. 

And  it  is  for  these  differences  that  in  optometry,  as  in 
other  specialties,  "Many  are  called  and  few  are  chosen." 


CHAPTER    XII. 

Resume  of  Previous  Chai'Ters  With  a  \ik\v  to  Emphasiz- 
ing THE  Salient  Points  of  Ocular  Skiametry  as  a 
System. 

The  definition  of  the  verb  "name,"  is,  to  fix  a  tliought  in 
a  word,  and  a  definition  of  "description"  is  the  act  of  depicting 
by  words  or  signs  so  that  another  may  form  a  correct  mental 
image  or  idea.  This  is  the  reason  why  the  name  "ocular  ski- 
ametrv"  has  been  given  to  eye-shadow--mcasuring.  as  set  forth 
in  Chapter  I. 

If  any  refractionist  can  find  a  better  name  for  the  test,  or 
series  of'  tests,  to  which  this  term  so  aptly  applies,  then  this 
name  can  be  easily  relegated  to  the  company  of  the  French 
word  "Fantoscopie-retinienne,"  as  suggested  by  Chibret.  or  ta 
the  manv  others  that  have  been  proposed  for  this  purpose. 

In  Chapter  I.  the  amount  of  optical  knowledge  necessary 
to  achieve  skiametric  success  was  called  attention  to.  Perhaps 
it  would  have  been  wise  to  have  also  included  a  knowledge  of 
higher  mathematics,  mechanics  and  medicine,  in  the  require- 
ments set  forth,  for  knowledge  of  almost  any  character  is  ot 
undoubted  value,  if  for  no  other  purpose  than  that  of  mental 
g>mnastics.  But  in  work  of  the  kind  referred  to  here  the  prac- 
tical must  be  kept  sight  of.  and  then.  too.  experience  counts 
for  much,  so  that  the  inexperienced  theorist  has  to  step  aside  for 
the  one  with  practical  knowle.lge.  even  though  this  knowledge 
may  be  gained  in  obscure  channels  and  unaccompanied  by  bnl- 
liancv  in  other  directions. 


Regarding  the  true  value  of  ocular  skiametrv.  in  order  to  be 
correct,  it  must  be  based  upon  recent  app.u.o.tment.  lor  every 
few  years  some  enterprising  intellect  designs  a  device  or  ofifers 
a  suggestion  that  betters  the  whole  system. 

Bowman's  discovery  of  the  shadow's  action  under  reflected 
light  was  similar  to  Babbage's  discovery  of  the  ophthalmoscope; 
inasmuch  as  neither  Bowman,  the  physician,  nor  Babbage,  the 
optician,  realized  the  full  or  even  partial  value  of  their  find 
until  masterminds  like  Cuignet.  Parent,  Helmholtz  and  others 
came  forward  and  advanced  the  work  which  has  already  bene- 
fited humanity  so  much. 

To  be  able  to  even  approximately  estimate  the  refractive  con- 
dition of  another  person's  eye  without  asking  a  question  is  in- 
deed marvelous,  but  to  make  this  estimation  with  exactitude, 
as  can  now  be  done  in  some  cases,  entitles  the  system  by  which 
this  is  accomplished  to  all  the  deference  due  to  "one  of  high  de- 
gree." 

Every  refractionist,  therefore,  should  give  such  a  systep" 
more  than  casual  consideration,  nor  should  he  be  satisfied  until 
after  he  has  thoroughly  investigated  its  merits,  consulting  only 
those  who  arc  its  masters,  for  it  logically  follows  that  those 
who  are  not  masters  of  it  are  hardly  qualified  to  render  intel- 
ligent opinions. 

.As  to  the  stumbling  blocks  in  skiametrv.  they  are  so  many 
and  varied  that  reference  to  tluni  will  he  by  chapters,  as  thev 
ai)i)ear  in  this  resume. 

In  Chapter  TT.,  attention  was  called  to  adequate  and  inade- 
quate examination  rooms.  Surely  work  that  requires  such 
attention  to  details  as  does  successful  optometry  also  requires 
attention  to  the  surroimdings  which  contribute  to  success. 
Practicing  refraction  work  in  an  inappropriate  place  is  like  an 
itinerant  watchmaker  erecting  his  bench  on  a  street  curb,  for 
even  if  he  could  manage  to  clean  a  watch  fairly  well,  the  public 


RESUMK  171 

is  not  to  be  blamed  for  lacking  confidence  in  the  conditions 
under  which  this  work  is  attempted. 

Regarding  illumination,  to  which  the  second  chapter  also 
refers,  it  can  perhaps  be  truthfully  said  that  poor  light  is  the 
cause  of  more  failures  in  ocular  skiametric  work  than  any  other 
one  defect. 

Good  work  usually  requires  good  tools,  and  a  gas  or  electric 
lamp  of  not  less  than  a  forty-candle  power  illumination  is  about 
the  poorest  light  that  can  really  be  called  good.  And  then,  too, 
this  light  needs  to  be  properly  hooded,  so  that  only  a  round 
aperture  of  about  three-quarters  of  an  inch  in  diameter  is  avail- 
able. 

As  to  the  kind  of  lamps  to  be  used :  the  gas  calls  for  the 
"Welsbach,"  or  incandescent  variety,  whose  mantle  should  al- 
ways be  in  perfect  condition,  while  the  electric  calls  for  a  car- 
bon filament  that  is  closely  formed  in  spiral  shape,  so  as  to  pre- 
sent the  appearance  of  a  small  solid  light  in  place  of  the  usual 
one  which  has  its  filament  wires  arranged  to  form  a  large  double 
ring  of  light.  Those  who  have  no  "city"  gas  or  electricity  will 
find  an  excellent  substitute  in  acetylene. 

The  one  point  to  be  remembered,  then,  is  to  be  sure  to  have 
a  good  powerful  source  of  illumination,  and  if  it  is  desired  to 
weaken,  or  lessen,  the  intensity  of  the  light  all  that  an  examiner 
has  to  do  is  to  operate  it  farther  away  from  his  patient. 

In  plane  mirrors,  that  which  is  needed  is  a  small  one  with 
a  two-millimeter  peep-hole  and  a  six-inch  handle,  the  silver- 
ing on  the  mirror  to  be  perfect  and  the  peep-hole  to  be  kept 
scrupulously  clean  and  free  from  dust. 

The  correct  handling  of  the  mirror  is  essential  to  skiametric 
accuracy.  The  bodv  movement  far  surpasses  a  hand  motion, 
because  it  permits  of  straight  line  action  in  place  of  curves. 
and  thereby  enables  an  examiner  to  make  closer  corrections  than 
where  the  mirror  is  allowed  to  wabble,  thus  causing  a  vertical 


motion  to  resemble  a  horizontal  one. 

Schematic  eyes  are  of  value  only  to  beginners,  or  for  experi- 
mental work.  However,  a  correct  one  is  necessary  to  obtain 
reliable  results,  and  reliability  of  findings  is  a  prerequisite  to  the 
encouragement  of  students,  for  if  a  novice  fails  to  obtain  accu- 
rate measurements  he  soon  tires  of  practice  and  fears  that  the 
system  is  at  fault. 

Transposition  of  lenses — "There's  the  rub,"  as  the  Bard 
of  Avon  puts  it.  To  be  sure  the  reduction  and  transposition  of 
lens  values  is  not  confined  to  ocular  skiametry  alone,  but  in  this 
work  it  is  found  of  especial  value. 

In  the  subjective  use  of  trial  case  lenses  transpositions  can 
be  proved  by  making  second  tests,  but  in  skiametric  work  this 
is  too  troublesome  and  also  wastes  too  much  valuable  time.  The 
successful  examiner,  therefore,  must  be  able  to  either  take  his 
pencil  and  figure  out  his  results  in  a  careful  manner,  or  else 
be  able  to  arrive  at  like  ends  through  mental  computations. 

The  method  for  reducing  and  transposing  lens  values,  as 
set  forth  in  the  second  chapter  of  this  book,  is  believed  to  be  the 
shortest  and  easiest  of  all  the  methods  that  have  been  devised 
for  this  purpose.  Its  salient  points  are  that  all  lens  quantities 
should  be  reduced  to  cylinders,  and  then  the  memorizing  of  two 
short  rules  governing  plus  and  minus  symbols,  which  show  that 
when  they  are  alike  the  cylinders  are  to  be  subtracted,  and  when 
they  are  unlike  the  cylinders  are  to  be  added,  the  cvlinders  al- 
ways being  considered  at  right  angles  to  each  other,  for  under 
no  other  condition  rlncs  a  rcfractionist  require  them. 

A  knowledge  of  tlic  rules  involved  in  this  system  enables  an 
examiner  to  juggle  with  all  kinds  of  lens  values,  no  matter 
whether  the  quantities  number  one  or  one  hundred. 

In  Chapter  TIT.  the  reader  is  asked  to  familiarize  himself 
with  the  optical  principles  involved  in  skiametry.  A  boiling- 
down  of  these  f)rinciples,  so  to  speak,  shows  that  the  real  object 


RESUME  173 

of  employing  the  shadow  is  for  the  purpose  of  measuring'  the 
exact  relation  of  the  lens  system  of  an  eye  to  its  retina. 

If  an  eye  is  seven-eighths  of  an  inch  in  depth  then  of 
course  it  requires  a  lens  system  to  have  a  seven-eighths-inch 
focus.  But  if  an  eye  has  a  seven-eighths-inch  focus  and  only 
six-eighths  of  an  inch  depth,  then  the  lens  system  must  be  arti- 
ficially assisted  by  adapting  spectacles  or  eye  glasses. 

In  trial  case-testing  the  optical  condition  is  determined  by 
cross-questioning  a  patient  as  to  vision,  while  rays  of  light  are 
being  bent  before  they  enter  the  eye.  In  examining  an  eye  by 
skiametry  its  optical  condition  is  determined  in  a  manner  in- 
dependent of  a  patient's  answers  by  noting  the  l)ehavior  of  the 
pupillary  shadow,  which,  in  turn,  represents  the  action  of  the 
rays  of  light  as  they  emerge  from  the  eye  under  extmination. 

In  subjective  tests  with  trial-case  lenses  the  operator  relies 
on  a  general  standard  of  measurement  when  the  object  looked 
at  is  situated  twenty  feet  away,  in  order  to  render  the  rays  of 
light  practically  parallel.  In  objective  examinations  by  ski- 
ametry the  examiner  deals  with  a  standard  in  which  conver- 
gence of  the  rays  takes  the  place  of  parallelism.  This  conver- 
gence, varying  as  the  distance  at  which  examinations  are  made, 
is  changed  to  meet  certain  other  requirements.  To  express 
it  tersely  then:  subjective  work  requires  the  following  of  a  ray 
of  light  to  its  retinal  focus,  while  objective  work  requires  the 
following  of  a  ray  of  light  from  its  retinal  focus.  In  subjective 
tests  the  source  of  the  ray  is  the  test  type  on  the  distant  card. 
In  objective  examination  the  source  of  the  ray  lies  at  the  retina 
of  the  patient's  eye,  and  at  a  point  which  is  represented  by  the 
line  of  demarcation  which  separates  the  illuminated  spot  from 
its  surrounding  shadow. 

Chapter  III.  also  describes  a  practical  and  easily  made  model 
which  will  undoubtedly  do  more  in  the  way  of  impressing  upon 
a  beginner  the  true  meaning  of  the  term  "shadow,"  as  used  in 


174  RESUME 

connection  with  skiametry,  than  many  pages  of  carefully  written 
discussion  could  do,  no  matter  how  intelligently  it  might  be 
expressed.  For  it  is  said  that  pictures  tell  a  better  story  than 
letters,  but  that  working  models  surjiass  both  letters  and  pic- 
tures. 

In  Chapter  I\'.  the  shadow's  behavior  under  certain  optical 
conditions  is  touched  upon,  and  a  description  of  regular  and 
irregular  refractive  errors  is  attempted.  Such  descriptions  are 
always  more  or  less  difficult  owing  to  the  fact  that  eyes  differ 
in  appearance  almost  as  much  as  faces  do.  For  this  reason  only 
the  more  pronounced  features  are  emphasized  here,  leaving  it 
to  each  examiner  to  differentiate  for  himself  as  his  experience 
increases. 

It  is  a  well-known  fact  that  those  who  are  interested  in  sci- 
entific pursuits  often  become  very  dogmatic  when  it  comes  to 
promulgating  theories  for  the  causation  of  certain  phenomena. 
Then,  too,  many  a  long  controversy  has  ensued  between  two 
theorists  which,  when  fully  understood  by  both,  was  found  to 
prove  that  each  one  was  in  the  right,  the  arguments  turning 
out  to  be  the  same  mental  picture,  only  taken  from  different 
points  of  view. 

The  theories  expressed  in  Chapter  \'..  while  having  no  prac- 
tical value,  except  to  account  for  disturbing  phenomena,  will 
perhaps  interest  those  who  like  to  delve  beneath  the  surface. 
Multiple  methods  for  using  skiametry,  however,  savor  very 
much  of  the  practical,  and  the  reader  is  here  introduced  to  the 
various  methods  for  applying  skiamctric  principles  which,  upon 
being  enlarged,  have  served  to  lift  Bowman's  discovery  to  the 
dignity  of  a  system. 

It  is  to  be  regretted  that  no  better  term  than  "non-toxico- 
static-method"  can  be  suggested  for  the  non-medical  rcfraction- 
ists'  manner  of  practising  static  skiametry  without  the  aid  of 
cycloplegics,  but,  to  repeat  that  which  has  been  said  before,  as 


the  nomenclature  of  optics  increases  {greater  care  must  l)c  ex- 
ercised in  selecting  words  accurately  descriptive. 

In  the  word  "fogging"  an  old  ac(|uaintance  is  met.  The 
word  amplifying,  however,  as  used  in  connection  with  other 
portions  of  optometry  besides  accommodation,  is  somewhat  new 
and  emphasizes  the  value  of  a  mobile  lens  system  over  that  of  a 
battery  of  single  lenses,  the  action  of  a  living  eye  itself  resem- 
bling the  former  rather  than  the  latter. 

Too  much  stress  can  not  be  laid  upon  the  usefulness  of  lens 
mobility  in  connection  with  ocular  skiametry,  owing  to  the  ease 
with  which  the  emerging  rays  of  light  can  be  bent,  and  this 
without  having  to  re-adjust  or  search  for  a  favorable  position 
every  time  a  lens  is  changed,  as  frequently  occurs  when  the  bat- 
tery, or  "unit,"  system  of  lenses  is  employed. 

In  Chapter  \T.  the  author's  new,  or  "dynamic."  method 
is  exploited.  And,  as  the  novelists  say.  "the  plot  thickens,"  for 
while  the  physical  and  physiological  principles  involved  in  ocu- 
lar skiametry  in  general  are  very  simple  indeefl.  when  once  un- 
derstood, yet  many  experienced  examiners  find  it  difficult  to 
grasp  this  new  method  at  a  glance,  and  are  therefore  prone  to 
think  that  the  system  is  at  fault  rather  than  their  own  under- 
standing. 

The  old  puzzle  picture  of  the  wild  cat  in  the  tree  will  i>os- 
sibly  serve  to  illustrate  this  point.  The  guesser  looks  at  the 
picture  intently,  but  can  see  only  an  ordinary  tree  having  leaves, 
twigs  and  branches.  He  keeps  looking  and  turning  the  picture 
in  all  directions,  when  lo!  there  is  the  wild  cat  outlined  by  the 
leaves,  twigs  and  branches,  and  as  plain  as  day.  only  a  little  dif- 
ferent in  appearatKe  from  what  he  expected. 

.\fter  once  being  found  this  wild  cat  never  can  he  lost  sight 
of  again,  and  the  beholder  wonders  how  he  ever  failed  to  solve 
the  puzzle  at  the  first  glance. 

So  it  is  with  dvnamic  skiametry,  after  the  principle  is  made 


176  RESUME 

plain,  that  the  patient's  own  accommodation  is  to  be  substituted 
for  that  of  the  working-distance  lenses  used  in  the  static  method, 
then  all  is  comparatively  easy.  It  must  be  borne  in  mind,  how- 
ever, that  this  accommodation  is  not  to  be  depended  upon  where 
it  does  not  exist  in  sufficient  strength  to  make  it  available.  For 
that  reason  patients  above  forty-five  or  fifty  years  of  age  are  to 
be  measured  by  the  static  method,  while  those  younger  are  to  be 
measured  by  the  dynamic. 

The  question  is  frequently  asked  by  students  of  the  dynamic 
method  whether  an  eye  is  emmetropic  or  not  when  an  examina- 
tion is  made  at  forty  inches  away  and  there  is  no  motion  to  the 
shadow,  the  patient  in  the  meantime  looking  at  a  fixation  card 
attached  to  the  examiner's  mirror. 

Now  this  tells  only  one  thing,  and  that  is  that  the  eye  is  not 
myopic  to  any  considerable  extent.  To  determine  whether  the 
eye  is  hypermetropic,  artificial  convex  lens  power  must  be  added 
to  see  whether  it  causes  the  shadow  to  move  against  the  mir- 
ror's motion,  and  the  strength  of  the  lens  necessary  to  cause  the 
reverse  movement  represents  the  amount  of  the  error. 

In  myopic  cases  the  myopia  must  be  over-corrected,  which 
really  makes  the  eye  falsely  hypermetropic  and  causes  the  ac- 
commodation to  be  exerted.  This  over-correction  is  then  meas- 
ured and  subtracted  from  the  concave  lens  quantity  that  has 
been  used  to  create  the  false  hypermetropia.  The  remainder  ob- 
tained represents,  of  course,  the  true  myopia. 

Astigmatism  has  been  described  as  a  half  error  and  the  re- 
fractive diflfercnce  between  its  weakest  and  strongest  points,  or 
limitations,  can  be  called  the  astigmatic  interval.  In  cases  of 
so-called  "mixed  astigmatism"  dynamic  skiametry  will  be  found 
of  especial  value  in  determining  whether  the  nearest  end  of  this 
interval  is  truly  myopic  or  is  only  a  spasm,  as  frequently  occurs. 

Once  more  let  it  be  said  that  dynamic  skiametry  is  not  an 
infallible  method,  because  it  depends  for  its  accuracy  upon  the 


RKSL'MI-:  177 

relation  existing  between  the  two  forces  represented  by  accom- 
mo(iation  and  convergence,  and  they,  in  turn,  depend  upon 
bodily  conditions  involving  nervous  impulses.  Hut  all  experi- 
enced refractionists  know  that  nine  prcsbyopes  out  of  every 
ten  require  about  a  two-diopter  convex  spherical  lens  to  restore 
the  harmony  between  this  same  accommodation  an<l  conver- 
gence when  a  patient  is  fifty  years  of  age. 

Therefore,  when  it  is  noted  that  the  basic  principle  upon 
which  dynamic  skiametry  is  founded  is  practically  the  same 
as  that  upon  which  Donders  compiled  his  famous  table  illustrat- 
ing amplitude  of  accommodation,  it  will  be  readily  seen  that 
one  will  bear  about  as  close  an  acquaintance  as  the  other  and 
be  found  just  as  reliable,  provided,  of  course,  that  "the  man 
behind  the  gun"  can  shoot  straight,  liesides  in  the  one  ca.se 
in  ten.  referred  to  above,  it  will  usually  be  discovered  that  it 
will  yield  to  the  non-toxico-static-method. 

Chapter  \'I.  referred  to  visual  fixation  and  hid  stress  u|K)n 
the  importance  of  obtaining  a  bright  fundus  reflex  before  satis- 
factory skiametric  work  can  be  accomplished. 

The  seemingly  half-hearted  way  in  which  many  would-be 
refractionists  go  at  their  work,  with  a  faulty  measuring  equip- 
ment, is  quite  enough  to  take  the  enthusiasm  out  of  any  one  who 
knows  the  full  value  of  paying  attention  to  details  and  of  having 
everv  possible  mechanical  adjunct  just  right.  And  here  it  can 
be  truthfully  .said  that  no  .skiametric  armament  is  complete  with- 
out a  fixation  stand,  or  its  equivalent. 

In  Chapter  \TI.  ociUar  muscle  action  and  the  influence  of 
habit  was  touched  upon.  Habit,  perhaps,  is  more  of  a  factor  in 
troublesome  refraction  cases  than  many  examiners  realize.  A 
typical  case  may  serve  to  illustrate:  Patient.  agc<l  fifty,  has  two 
diopters  of  simple  hypermetropia  and  two  diopters,  of  course, 
of  presbyopia.  Has  been  wearing  three  diopter  convex  spherical 
lenses  for  general  housework  and  .(.iiiDl.iins  of  Ixith  poor  vision 


178  kitsUMK 

and  "distress  in  the  head."  Two  pairs  of  glasses  are  given, 
patient  returns  in  a  few  days  and  both  reading  and  distance 
glasses  are  weakened  to  relieve  an  "unpleasant  feeling."  An- 
other week  and  another  slight  change  is  made.  Still  another 
return  and  the  original  glasses  are  again  given.  Report  in  a 
month  is.  that  the  glasses  are  "very  satisfactory." 

Xow  the  logical  conclusions  to  be  arrived  at  in  this  case 
are  that  "you  can  not  teach  old  dogs  new  tricks"  in  a  hurry — 
nor  some  young  ones  either  for  that  matter.  Old  habits  of  ac- 
commodation, convergence  and  innervation  had  to  be  broken 
up,  and  new  ones  formed  again  before  comfort  was  obtained, 
and  this  reformation  consumed  considerable  time,  for  the  old" 
habit  Mas  one  of  long  standing. 

The  insistence  upon  a  patient's  perseverance,  however,  when 
the  glasses  were  not  comfortable  has  cost  many  an  examiner 
much  annoyance,  so  the  old  adage  "be  sure  you  are  right  and 
then  go  ahead"  is  a  good  one  for  a  refractionist  to  adopt. 

As  has  been  shown,  habits  of  accommodation  are  responsible 
for  habits  of  convergence,  and  both  manifest  themselves  in  dis- 
turbed, unequal  and  excessive  innervation.  So  it  can  readily  be 
seen  that  the  adaptation  of  prisms  should  be  postponed  until  the 
correction  of  an  error  of  refraction  has  had  time  to  cause  the 
formation  f)f  new  habits  of  rekition  between  accommodation  and 
convergence. 

In  Chapter  \TII.  corroborative  measurements  in  optometry 
are  strongly  urged.  In  fact,  objective  and  subjective  optometry 
are  now  found  to  be  as  dependent  upon  one  another  as  a  knife 
and  fork  are.  To  be  sure,  either  one  can  be  struggled  with 
alone,  and  a  measure  of  success  achieved,  but  for  all-round 
optomctrical  work  no  one  method  can  be  implicitly  relied  upon. 
Ignorance,  malingering  and  carelessness  on  the  part  of  the 
patient  are  a  few  of  the  causes  that  militate  against  the  relia- 
bility of  subjective  tests,  while  ignorance,  lack  of  skill  and  care- 


RESUME  179 

lessness  on  the  part  of  an  examiner  are  corres|)on(ling  factors 
that  stand  in  the  way  of  successful  objective  results. 

In  corroboration  lies  the  only  safe  way.  The  i)hysician  who 
depends  solely  upon  his  trial  case  or  upon  to.xico-static  ski- 
ametry  is  sadly  crippled.  And  the  optician  likewise  who  do-, 
pends  solely  upon  his  trial  case  or  ujion  non-toxico-static  ski- 
ametry  is  even  worse  off.  To  be  an  expert,  a  refractionist  must 
be  master  of  all  methods  and  systems  that  can  lay  claim  to  merit, 
no  matter  whether  this  claim  be  a  larg;e  or  a  small  one. 

Regardinjr  the  value  of  instruments  in  ocular  skiametry.  the 
universal  opinion  seems  to  be  that  all  devices  which  have  been 
placed  upon  the  market  for  ijse  in  shadow  testing  ap[)ear  to  be 
possessed  of  more  or  less  usefulness,  and  that  the  device  which 
will  hve  longest  and  achieve  the  greatest  popularity  will  be  the 
one  having  the  greatest  number  of  points  of  excellence  in  its 
favor. 

A  skiametric  instrument,  in  order  to  meet  present  demands, 
must  be  so  made  that  its  use  is  both  easy  and  rapid  as  well  as 
accurate.  This  ease  and  rapidity  are  not  so  nuich  to  save  an 
examiner's  own  time  and  trouble  as  to  encourage  him  in  the 
use  of  skiametry  in  all  of  his  cases,  so  that  he  may  become  pro- 
ficient in  the  skill,  which  is  such  a  vital  part  of  successful  ski- 
ametric work. 

If  an  instrument  is  clumsy  of  adjustment  or  requires  a  pa- 
tient to  be  moved  from  one  place  to  another  in  order  to  use  it 
an  examiner  will  find  himself  skipping  occasional  cases  and 
guessing  at  conditions.  And  this,  in  turn,  leads  to  carelessness 
with  its  train  of  evil  consequences.  So  the  examiner  who  insists 
on  getting  the  very  best  in  the  way  of  instruments  and  general 
examination-room  armament  will  usually  find  himself  some  day 
lined  up  in  the  ranks  with  the  successful  ones  in  his  calling. 

Chapter  IX.  deals  with  the  subject  of  the  relative  value  of 
skiametric  insirnmenis.  and  their  evolution  from  single  trial- 


i8o 


case  lenses,  etc.  It  is  said  to  be  quite  natural  for  parents  to 
think  well  of  their  own  offspring,  so  it  is  to  be  presumed  that 
due  allowance  will  be  made  for  the  author's  confidence  in  the 
superiority  of  the  mechanical  achievements  upon  which  he  has, 
off  and  on,  spent  nearly  fourteen  years  of  thought  and  labor. 
Hence  it  can  be  readily  seen  that  if  genius  had  taken  the  place 
of  industry  this  term  of  years  might  have  been  materially 
shortened.  But  as  the  system  explained  in  these  pages  repre- 
sents an  evolution  that  is  based)  upon  experience,  so  do  the 
author's  mechanical  devices  represent  a  similar  result.  There- 
fore an  understanding  of  the  former  leads  to  an  appreciation  of 
the  latter,  as  the  various  devices  were  designed  expressly  to  fit 
the  system. 

Instruments,  however,  like  refractionists,  must  stand,  sooner 
or  later,  upon  their  own  merits,  and  therefore  all  that  any  manu- 
facturer of  optometrical  apparatus  can  reasonably  ask  is  that  his 
products  be  accorded  an  intelligent  and  trustworthy  trial  of  suf- 
ficient duration  to  not  only  develop  the  superior  qualities 
of  his  devices,  but  to  develop  the  ^^;7/  of  the  user  of  them  as 
well. 

Chapter  X.  covering  systematic  ocular  examinations  and  the 
recording  thereof,  and  Chapter  XL,  with  its  illustrative  cases, 
showing  the  superiority  of  the  dynamic  method  over  that  of  the 
static,  under  certain  conditions,  etc.,  are  both,  no  doubt,  too 
fresh  in  the  mind  of  the  reader  to  justify  a  repetition,  even  of 
their  salient  points.  The  general  aim,  however,  of  the  preceding 
chapters  has  been  to  impress  upon  those  interested  that  the 
essentials  to  success  in  ocular  skiametry  lie  along  many  paths, 
and  that,  to  compactly  summarize  them,  they  can,  perhaps,  be 
reduced  to  nine  in  number,  as  follows : 

First.  An  adequate  examination-room. 

Second.  A  good,  strong,  properly-hooded  light. 


i8i 


Third.  Not  too  large  a  plane  mirror,  cither  in  area  or  peep- 
hole. 

Fourth.  Correctly-placed  fixation  points,  or  cards. 

Fifth.  An  accurate,  binocular  mobile  lens  system. 

Sixth.  Skill  in  detecting  the  action  of  the  shadow. 

Seventh.  Ability  to  reduce,  transpose  and  combine  lens 
values. 

Eighth.  Knowledge  of  how  to  master  tonic  and  clonic 
spasms. 

Ninth.  Differentiation  of  regular  and  irregular  cases. 

It  is,  therefore,  sincerely  to  be  hoped  that  this  resume  has 
been  made  both  sufficiently  interesting  and  instructive  to  war- 
rant the  virtual  thrice-telling  of  the  same  tale. 


of  the  Alamena 
Gouniy  AssoGiatioa 
of  UpK.metrists 

INDEX 


Page 

Accommodation    99,  1 13 

Inriuence  of 120 

Acetylene    lamps 29 

Action  of   sliadow 69 

"Against"  the  mirror ^2 

Altering  pupillary  distance.  141 

Amplifying    method 95 

Appearance  of  shadow 80 

Argand  Lamp 28 

Astigmatism    76 

Astigmometry   11 

Babbage  170 

Band  in  astigmatism 81 

double    84 

"      oblique    axis 82 

Bowman   11,16 

Cases,    illustrative 154 

Chibret    169 

Chimneys    2>^ 

Chromotometry    11 

Compound    error "/j 

Conjugate   focus 72 

Convergence,  influence  of. .  120 
Corroborative            measure- 
ments    124 

Crain    disc 132 

Cross  retino-skiamcter 137 

Cuignet    170 

Cylindrical  equivalent 84 

Dark-room   24 

Difficulties  of  skiametry.  ...  19 

Dioptometry    11 

Discovery,  Bowman's 11 

Donders   163 

Dull   reflexes 87 

Dynamic  skiametry 98 

Electric  lamp 35 

Emmetropia    -j}^ 

Examination-rooms    24 

"             systematic  ...  147 

Fantoscopie-retinienne    ....  i6f) 

Fay  instrument 134 

Fixation   points 106 

Fixation    stand 107 

Focused    light 15 

Fogging  method 95 

Fundus  reflex 87 

Gas    lamp 28 

Gasolene  lamp Z}, 


Page 

Geneva   retinoscope 136 

Habit,  influence  of 120 

Hamilton  refractometer.  . . .  134 

Hartridge   99 

Helmholtz    170 

Holding  the  mirror 41 

Hypermetropia    74 

Illumination    26 

Illustrative    cases 154 

Influence  of  accommodation  120 

"  convergence...  .  120 

"  habit   112 

Instruments  of 

Crain   13-2 

Cross   137 

DeZeng   36 

Fay    134 

Geneva    '    136 

Hamilton    134 

Jennings    133 

Meyrowitz    134 

Prentice   136 

Standart    132 

Wiirdemann    131 

Irregular  errors 84 

Jackson    5,  136 

Jennings'    Device. 133 

Keratometry   11 

Lamps    26 

Acetylene    29 

.•\rgand    28 

DeZeng    28 

Electric   35 

(iasolcne    2,2, 

Oil    26 

Rochester    26 

Success    27 

Welsbach    30 

Lenses,  reduction  of 44 

"         transposition  of...  .  44 

mobile   action 128 

Light 26 

reflections   139 

Magnified    punil 144 

Meyrowitz   refractometer..  .  134 

Mirror,    plane 38 

"         bracket    39 

Mixed  astigmatism "j^ 

Mobile  lens  action 128 


INDEX      Continued 


Page 

05 
4^ 
93 

112 
75 
II 
94 

152 
M 


Mudel  for  shadow 

schematic   eye 

Multiple  methods 

Muscle  action  and  habit... 

Myopia    

Name   descriptive 

Non-toxico-static  method. 

Nystagmus  

Objective  method 

Ocular  skiametry 1.3 

Discovery  of 16 

Principles   of 50 

The  shadow 64 

Amplifying 95 

Fogging   95 

Dynamic    98 

Static  93.  100 

Ophthalmometry     11 

Ophthaimotometry 11 

Ophthalmotropometry   

Optometry    

Parent    

Penumbra,   single 

"  double    

"             in   shadow-test- 
ing   

Plane    mirror 

Position  of  light 

"  "  mirror     

Perioptometry   

Phacometry    

Phorometry    

Prentice  retinoscope 

Prisoptometry   

Prisms,  use  of 

Pupillary   distance 

Pupillomctry    

Ray  values 

Records     

Reduction  of  lenses 

Regular   errors 

Resourcefulness    

Resume 

Retinal  illumination 

"        reflex       

Retinoscopy    

Retino-skiameter    

Risley    prism 


Roentgen   

Schematic    eye 

Shadow    

Action  of 

Models  for  demonstrat- 


11 
II 
170 
91 
92 

93 


40 

II 

II 

II 

136 

II 

121 

141 

II 

no 

148 

44 

81 

152 

169 

63 

63 

12 

1.17 

127 


ing    

Movements  of. 


Appearance     in     spher- 
ical cases 

Appearance     in     astig- 
matic cases 

Appearance      in      com- 
pound cases 

Appearance  in   scissors 
movement  

Appearance  in  irregular 
astigmatism    

Appearance   in   cortical 
cataract   

Appearance    in    conical 

cornea    

Shadow  test 

Skiametry    

Skiameters    

Skiascopy   

Skiascope    

Spasms    

Standart   disc 

Static   skiametry 

Strabismometry   

Subjective   method.. 

Systematic    examinations... 

Theories    

Thorington    

Toxico-static  method 

Transposition  of  lenses 

Using    skiameter 

Value  of  Bowman's  discov- 
ery   

Value  of  instruments 

Vision  of  examiner 

Visual   fixation 

Visuometrv   

Wall    bracket 

Welsbach    

"^V'ith"  the  mirror 

Wisdom    

Wiirdcmann's  lens  rack 


Pace 
'3 
42 
64 
70 

6S 
70 

81 

81,82 

83 

8U 

&4 

85 

85 
13 
13 

12 

38 

"7 

132 

93. 100 

II 

14 

147 

f^7 

94 
48 
142 

16 
1^7 

23 
105 


7! 
113 
131 


14  DAY  USE 

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